Episode 3x04 -- "Disturbing Behavior"
"There's a girl with the necklace. You love her. You'd do anything for her and you have. A lot of darkness, a lot of guilt." ~ Gloria
This scene doesn't involve Elena directly, but in flashbacks during Gloria torturing Stefan we see how much he loves her. The witch is capable of feeling everything he feels for the doppelganger and verbalizes it. Being able to see and hear Stefan's emotions really heightened the importance of this scene.