Elena and (half-)naked!Damon. Let’s just talk about the development for a second…
1x15 - Damon’s naked body has little to no effect on Elena, she is annoyed when he invades her space and doesn’t even seem to acknowledge that he’s half-naked.
3x01 - Elena is already considerably more flustered by Damon’s nakedness, especially after seeing him full-on naked. Especially after sharing even the smallest kiss in 2x22, she is more and more affected by him, and his (naked) proximity.
3x19 - Unbeknownst to Damon, Elena is openly admiring Damon, his physique just as Damon as a whole. She is fascinated by him, attracted to him, overwhelmed by him and well, it must be a nice plus that his sexy chest is showing right there, lol.