Jared Padalecki was a major part of Gilmore Girls' formative years, where he played Rory's first boyfriend, Dean Forester, but he suddenly left the series in season 5. The love-life of Rory Gilmore (Alexis Bledel) was key character in Gilmore Girls, and a hot-topic among fans, with frequent debate over which of her on-screen boyfriends (if any) she truly belonged with. While fans typically argued more for Jess Mariano (Milo Ventimiglia) or Logan Huntzberger (Matt Czuchry), Dean was still an important part of the show.
The character first appeared in the pilot, with the original Dean actor being recast as Padalecki replaced Nathan Wetherington after the first version of the episode required several changed. He was then a crucial figure across the first three seasons, with much drama wrought from his and Rory's relationship (and the road-bumps therein). Dean started to take more of a backseat in season 4, dropping from a main to a recurring role, and that continued for part of season 5 - before he left altogether.
Dean and Rory's story came to an end in Gilmore Girls season 5, episode 8, "The Party's Over". At that point, having sat waiting for Rory while she partied with people from Yale (including Logan), Dean realized that he no longer fit into Rory's life and they were better off apart. Dean later made one more appearance in the show's original run, guest starring in season 5, episode 18, "To Live and Let Diorama", where he tries to offer Luke advice on dating a Gilmore, and the pair end up butting heads. While there was no real explanation to Luke's departure, the reason he wasn't seen again is because Padalecki took on the starring role in Supernatural as Sam Winchester, which he's been in ever since.
Funnily enough, although Padalecki's role on Gilmore Girls was his most high profile before becoming one half of the Winchester brothers, it did almost cost him the part. One of the key qualities Supernatural creator Eric Kripke wanted in Sam was for him to be highly intelligent, which, in fairness to Dean, was not something he showed. Padalecki told TV Guide of his manager's initial conversation with Kripke: "Eric's like, 'Uh, I've seen Gilmore Girls. We want Sam to be really smart.' And so my manager had to go, 'Uh, well, my client is a National Merit Scholar.'"
Exactly what happened to Dean after that point isn't fully clear, though he's presumably still around Stars Hollow for some time after season 5. Padalecki made a brief return to the role in 2016's revival Gilmore Girls: A Year in the Life during the final episode "Fall". There, he catches up with Rory, and it's revealed that he now lives in Scranton, Pennsylvania with his wife, Jenny, with whom he has three children (and she's pregnant with a fourth). With around a decade between the end of Gilmore Girls and the start of A Year in the Life, there's plenty of time for Dean to have left and started a new family.
While it's understandable that Padalecki had to quickly leave Gilmore Girls for Supernatural, which started airing in 2005, it is a shame that he wasn't given a more definitive send-off after how big a part of the show he'd been. That, at least, was something his return in A Year in the Life, despite its brevity, was able to do, providing a fitting and touching conclusion to his and Rory's relationship.