Why Does Elena Become a Vampire?

Trapped under water, Elena sacrifices her life to save Matt's, just as her father had sacrificed his life to save Elena the night Stefan rescued her from her family's submerged car. ... When Elena died, she had the vampire blood in her system, meaning that Elena is now a vampire.

Who turns Elena turn into a vampire?

Elena dies with Damon's blood in her system, triggering her transition. The first episode of season 4, "Growing Pains," features Elena coming to terms with her transformation. Against Bonnie's efforts, the doppelganger is forced to drink human blood and complete her change into one of the undead.

Why does Elena choose Damon?

Damon doubted her feelings for him when it was discovered she was sired to him, therefore had to please him. After Elena lost her humanity, she taunted Damon, threatening to go back to Stefan since the sire bond was gone. However, after she regained her emotions, she confessed her love for Damon, choosing him.