The Vampire Diaries Season 1 Episode 20 -

Throughout the series, Damon has been a wild card—violent, unpredictable, and driven solely by his obsession with Katherine. However, in this episode, we see the first crack in that persona. When Stefan is gravely injured protecting Elena, Damon doesn't flee. He stays. He admits, albeit through gritted teeth, that despite 145 years of hatred, "You’re still my brother."

“You two are the most infuriating, self-destructive, codependent pair I have ever met. But when you work together… you’re unstoppable.” the vampire diaries season 1 episode 20