Sam and Dean have been both lucky and unlucky in love. While they do seem to find love or romance fairly easily, the romance often doesn't last long. Sometimes, it's cut drastically short due to their lifestyle, "saving people, hunting things, and the family business."
Other times, their romances are drastically cut short because the woman was killed. For a while, this seemed to happen to most women the brothers were involved with, leading the viewer to hope that the romance wouldn't happen so that the women would live. Still yet, there were romances that went on too long, making us uncomfortable.
10 Ended Too Soon: Sam and Sarah
In Season 1, Sam meets Sarah, the daughter of the art gallery owner in possession of a haunted painting. Sarah puts herself in harm's way when it comes to this case. She feels responsible for the recent deaths since she and her father were the ones who sold the painting.
Sarah is intelligent, leading to witty banter with Sam. In addition, she refuses to let Sam make decisions for her. When he says that he wants to protect her from his lifestyle, she tells him that isn't his decision to make - that she can make her own decision. Ultimately, Sam isn't over Jessica yet, so this romance is short-lived. Still, Sarah was the first person he liked after Jessica's death, which says something about their connection.
9 Didn't End Soon Enough: Sam and Amelia
In Season 8, Sam quits the family business, doesn't help prophet Kevin, and doesn't try to get his brother out of Purgatory (where Dean has been since the end of Season 7). What brings about this change of heart? In one word - Amelia, he falls for a vet. Amelia represents the normal life that Sam has always craved.
Sam has a normal life, doing maintenance work, owning a dog, and living with Amelia, someone who doesn't know his other life at all. While it seemed that he did care for Amelia, and perhaps both did love each other, the relationship lacked the chemistry of his previous ones. In addition, he didn't share all of himself and betrayed his brother - making this romance not sit well with viewers.
8 Ended Too Soon: Dean and Cassie
Dean met Cassie before he and Sam teamed up again. Their first meeting and relationship are off-screen, but it's clear that there was love and passion. When Cassie called Dean to ask for help, Dean dropped everything to come to her aid.
When Sam watches them, he notices that they won't look directly at each other, but when they think that the other one isn't paying attention, they can't help but look at each other. Sam also learns that while he kept his family secret from Jessica for years, Dean quickly told Cassie once they began a serious relationship. In the end, Cassie and Dean realize that they can't be together due to his hunting lifestyle.
7 Didn't End Soon Enough: Dean and Anna
Dean and Anna were very briefly with each other, in a steamy car session that rivals Titanic's. They had a connection, leading Dean to protect Anna. She, an angel who decided to fall from heaven in order to become human, represents something unusual to him. He asks her why would you want to be human. She tells him that she wanted to feel.
Later, in another episode, Anna turns deadly. In order to get back into good graces with heaven and God, she goes back in time and tries to stop the brothers from being born by killing their parents.
6 Ended Too Soon: Sam and Madison
In Season 2, Sam and Dean investigate werewolf killings. They discover that Madison was bitten by a werewolf and is now transforming. Sam stays with Madison a few times in order to first protect her and then to protect others from her. Their connection becomes physical, and they have lots of chemistry.
However, as much as Sam tries to reverse the werewolf curse, he can't. Ultimately, she asks him to kill her, and he does. The hard part is in later seasons, they learned how to reverse the werewolf curse when Claire (Jimmy's daughter) becomes infected in Season 12. If they had that knowledge in Season 2, Madison may have been to live to another day and be Sam's romantic interest for a little longer.
5 Didn't End Soon Enough: Dean and the Darkness, Amara
Dean releases God/Chuck's sister, and she seeks to destroy nearly all of her brother's creation (except Dean). Amara enters Earth as a baby who sucks souls, and she quickly grows up. When she is a teenager held by Crowley, she gets almost giddy at the mention of Dean's name, and ultimately, she protects Dean from Crowley.
When she is grown and fully at her powers, the two continue to be drawn to each other. At a moment, she goes to take Dean's soul, but she ends up kissing him instead. While this pairing was interesting, it just went on a little too long with few additional dynamics, making it seem flat after a while.
4 Ended Too Soon: Dean and Bela
While these two didn't really start a relationship, there was a hint of one beginning. Although Sam may have been attracted to Bela as well (shown by his dream), Bela was more attracted to Dean. When they dress up for a case, Bela can't help but admire how good-looking Dean is.
Additionally, when she has to call the brothers, it's always Dean that she calls. Unfortunately, Bela was too jaded at this point to be any good in a relationship; however, we would have liked to see her try.
3 Ended Too Soon: Dean and Jo
Jo, a fan favorite, was a perfect match for Dean. Not only was she smart, but she also grew up as the child of a hunter. Jo also is a hunter, which means that she understands the life.
Added to that, there is genuine affection between Dean and Jo. While Dean had a longer relationship with Lisa, and he was good with her, his chemistry with Jo always seemed stronger. We would love for her to come back in Season 15, and for Supernatural to give their romance a chance.
2 Didn't End Soon Enough: Sam and Ruby
While Dean was in hell, Sam started a romance with a demon from hell--Ruby. When he finally tells Dean the truth (once Dean is resurrected), he spares no details, which leads Dean to say, "Sam, too much information...now I feel dirty." We agree with Dean.
The fact that Sam and Ruby went on for a long time, that she encouraged him to drink more demon blood, and that she manipulated Sam through it all doesn't speak well of their romance. But then, again, she is a demon. We'd expect nothing else.
1 Ended Too Soon: Sam and Jessica
Sam lived with Jessica, and they had been together for a while. She came across as both a strong and nurturing person, and Sam was going to propose to her. However, he never told her about his family history, wanting to keep her out of it.
Jessica was the first casualty--due to her connection to Sam, she died. While we wanted more time for Sam and Jessica, it's also clear that had she not died, Sam wouldn't have been motivated to begin hunting again with Dean.