In The Vampire Diaries, Elena and Damon found their way to each other, got married and started a family together but there were several times that both Elena and Damon treated each other badly.
It was manipulative
With her humanity off after Jeremy’s death in The Vampire Diaries, Elena was not a very nice person. She was much more lively and fun, but also manipulative and dangerous.
She mainly tormented Stefan during this period of her life, but Damon also ended up being the victim of her cruel words. She, for example, made fun of their shared past and said she never loved him on The Vampire Diaries.
Elena erased her memories of Damon
Instead of dealing with his death like a sensible person, Elena from The Vampire Diaries did not respect his memory, erasing from all her memories of Damon and how much she loved him.
Put Damon in danger
In the second season of The Vampire Diaries, Elena chooses to continue with the ritual and not run from Klaus, who wants to sacrifice her, and a vampire and a werewolf in the ritual.
How did this put Damon in danger? Well, Klaus is as unpredictable as he sounds and he needed a vampire, so Damon was one of the most obvious choices in The Vampire Diaries.