Stefan and Elena, the original couple in The Vampire Diaries, broke up in season 4 due to Elena's feelings for Damon. Elena falls in love with Damon after her transformation into a vampire and relies on him instead of Stefan. Stefan's behavior also contributes to their breakup, as he treats Elena like a project instead of a person.
While their romantic relationship ends, Stefan and Elena maintain a meaningful friendship throughout the rest of the series. They work together to find the cure for vampirism and Elena helps Stefan regain his memories after Qetsiyah affects his brain.
Stefan eventually falls in love with Caroline, and Elena supports their relationship.
Though The Vampire Diaries season 4 marks the end of Stefan and Elena's romance, they still have a meaningful platonic relationship after their breakup. The pair work together to find the cure for vampirism.
When Qetsiyah fries his brain, Elena spends time trying to help him get his memories back. Stefan develops feelings for Caroline, and Elena encourages the romance. While he starts out against them, Stefan also helps Damon and Elena find their way back to each other after Damon returns from the prison world.