Dean and Jack

The relationship between Dean Winchester and Jack is a complicated one. Due to Jack being the son of Lucifer, Dean originally viewed Jack as a threat that had to be stopped from being born. After Jack was born, with Sam unwilling to kill him, Dean would keep an eye on Jack, threatening to kill him if Jack ever turned evil. Jack in turn sought Dean's approval and affection.

Over time, Dean and Jack's relationship evolved to be that of a father and son with multiple characters noting the parental bond between the two. At one point, God himself compared Jack dying at Dean's hands to be akin to a father killing a son. When Jack was dying, he chose to do things with Dean that Dean had enjoyed doing with his own father. Despite his words and his anger over Jack killing his mother, Dean's love for Jack was strong enough for Dean to spare Jack's life and mourn Jack's death when God killed him moments later.

Background
Dean originally viewed Jack as a dangerous being and blames him for the loss of Castiel, as Jack had been the one to convince Castiel to watch over his pregnant mother and leave Sam and Dean behind. Dean spent the months leading up to Jack's birth feeling upset and angry and viewed Castiel's running away with Kelly as the angel being manipulated.

Season 13
When Jack was finally born, Dean's first response was to shoot him, only for Jack to unleash his powers in self-defense and knock Sam and Dean out. Dean's opinion on Jack didn't improve as he began hunting for the Nephilim. His distrust in Jack contrasts Sam's softer opinion, likely due to Dean's grief caused by losing Castiel again.

Dean does however, give Jack's mother a hunter's funeral alongside Castiel.

Jack soon learns that Dean still doesn't like him and makes a clear effort to emulate Dean in order to appear more likable. Dean is both annoyed and distrustful of Jack. Dean eventually admits to Jack that should Jack go dark side as Dean anticipates, he'll kill Jack himself. Jack overhears an argument between Dean and Sam in which he learns of Dean's negative feeling stem from him blaming Jack for the death of Castiel. In turn, Jack sometimes believes he is evil because of Dean's words, but Sam does his best to cheer Jack up.

"He manipulated him. He made him promises, said paradise on Earth, and Cass bought it and you know what it got him? It got him dead. Now you may be able to forget about that, but I can't!"

- Dean's initial opinion on Jack

Jack continues to long for Dean's approval while Dean chooses not to baby him and make him do some of the work during cases, or in Sam's words, treat him like an intern. When Jack becomes dismayed by the lack of approval from the elder Winchester and keeps thinking about Dean's promise to kill him, Sam informs Jack that he can earn Dean's approval through effort, something Dean respects.

While confronting the shapeshifter Buddy, Dean and Jack are both chained up though Dean orders Jack to use his powers to free them. Jack, afraid his lack of control over his own powers may hurt Dean, refuses to obey the order. Jack instead uses his powers to prevent Buddy from shooting at Sam, which gives Sam the opportunity to shoot Buddy, thus solving the case they were working on.

Afterwards, Dean praises Jack for his efforts and acknowledges that he finally did good as Jack smiled to himself at this praise.

With Castiel's resurrection and return, it appeared Dean gave up some of his disdain for Jack since he had partial role in it and he listed Jack as a member of the team. Dean even counted him as a member of Team Free Will and humored Jack on a hunter case but was ready to go along with as it allowed him to act like a cowboy. He was also surprised that Jack was right about it being a case though the monster was something else. Jack wanted to be useful and tried to help Dean by using his powers though unintentionally killed an innocent. Rather than scold Jack, Dean instead told Sam and Castiel to take him back to the Bunker while he deals with the threat. After completing the case, Dean even covered for Jack by putting the innocent's death on the monster. Jack was shown not to accept this and broke down that he might actually be evil but Dean said he wasn't as he admitted that he was wrong in thinking he was.

"Jack: I was scared, I was upset, but why would I look for him? He's no one to me. You, Castiel, you're my family. Dean: Yes, we are."

- Jack and Dean

After learning that Jack apparently killed Derek Swan, Dean discussed killing him for it with Sam. However, Dean admitted that he actually liked Jack and saw it as a worst-case scenario that he must be killed. Once they found him, they confronted Jack over his action but he was revealed to be innocent and told them his true intention were to find their mother, which he was able to do letting them know their mother is still alive. When Jack called the Winchesters and Castiel his family, Dean instantly agreed that Jack was family. Dean then praised Jack's initiative in trying to rescue Mary as the latter smiled at his words. Dean continued to be this way when Jack refused to join the angels and wanted to stay with them instead.

During Devil's Bargain, Dean and Sam are told by Castiel that Michael of that world intends to invade. Dean realized Jack and his mother are in danger and they decide to capture Lucifer and open a rift to enter the Apocalypse World to save the pair. Dean shows he still sees Jack as part of the family.

In Good Intentions, Jack chooses to help defend Bobby Singer's colony from Zachariah and his angels because he knows its what Sam and Dean would do. He also knows that Sam and Dean would stay and fight and chooses to kill Michael to protect the people of Apocalypse World.

In Beat the Devil, Dean and the others open a rift and travel through Apocalypse World.

After finding Mary who takes them to a camp, Dean reunites with Jack and both witness Sam's return as well as the arrival of Lucifer.

In Exodus, Dean told Jack to not approach Lucifer as he didn't want Jack to turn evil.

During Let the Good Times Roll, Jack had a nightmare but Dean came in his room to comfort him on this and told him they were family. Jack went on more cases with Dean and the others. The bunker was attacked by Michael and Jack was alerted of this by Sam's prayer. He arrived in time to save Dean from nearly being strangled to death by Michael and almost acted on his promise to kill the Archangel but was prevented when Michael revealed his deal with Lucifer. Jack was later horrified to watch Michael take full control over Dean after he managed to kill Lucifer. Weeks later, Jack's frustration over his lack of powers is also attributable to him not being able to do anything to help Dean.

Season 14
During Gods and Monsters, Jack is still worried about Dean's possession. Jack is shown to have a rational side such as telling Castiel that to stop Michael, then Dean had to die since he knew the latter wouldn't want to suffer from Michael's influence.

During The Scar, Jack is reunited with Dean who he is happy to see and hugs out of joy but questions him on what became of Michael. While talking to a girl who was cursed that asked if Castiel was his father, Jack responded that he was one of them. This means he sees Dean as one of his father figures as well.

In Unhuman Nature, its confirmed that Jack and Dean's relationship has evolved to that of a father-son relationship. Sam tells Castiel that Dean has never forgiven himself for threatening Jack in The Rising Son and Castiel compares Dean's impending loss of Jack to death to a father losing a son. In order to help Jack get enjoyment out of his remaining days, Dean teaches him how to drive using the Impala and Jack has Dean take him fishing, something that Jack knew was the one thing that Dean had enjoyed doing with his own father.

In Byzantium, Dean is so devastated by Jack's deteriorating condition that he has to leave the room when Jack tries to comfort him and is further devastated that Jack dies while he's gone. While trying to get Lily Sunder to help save Jack, Dean explicitly calls Jack his son, asking Lily to not force them to go through what she did when she lost her child. Dean's impassioned plea succeeds in convincing Lily to help them save Jack who is happily embraced by Dean upon his resurrection.

After Jack accidentally kills Mary in Game Night, Dean reverts back to his original opinion that Jack is a monster that needs to be killed, albeit only because Jack no longer has his soul. Though Dean advocates for first locking Jack up in the Ma'lak Box and then killing him, Dean is clearly not pleased to have to do so though he finds it necessary and is driven at least partially by a need for revenge.

"I understand. I know what I've done. And you were right, all along. I am a monster."

- Jack to Dean

In Moriah, Dean prepares to kill Jack with a gun supplied by God despite the fact that it will also cost Dean his life. Dean refuses to be dissuaded, even by Sam who points out that Jack is in this position because he burned his soul up killing Michael to save them. When the time comes, Jack accepts his fate and even tells Dean that he was right all along and Jack is a monster. However, when it comes down to it, Dean is unable to kill Jack, even when God offers to resurrect Mary if he does so. God compares Dean killing Jack to a father killing his son and is enraged by Dean's refusal to do so, finally killing Jack himself. Dean is left devastated by the loss of Jack alongside Sam and Castiel, but is left with no time to mourn due to God unleashing chaos upon the Earth.