My ex girlfriend is dating another guy
Elena has admitted she sees the good in Damon and believes he's worth saving.
Elena begins to reciprocate Damon's feelings for her, though continues to love Stefan.
Damon has never lied to her and would always tell her the truth, although he has admitted that he will lie to her if it saves her life.
After becoming a vampire, Elena breaks up with Stefan as her feelings for Damon had become too strong for her to ignore.
Though initially started on antagonistic terms, their relationship develops throughout the series.