Imagine: Peter talking to Derek about you when he sees you.

Derek had been teasing Peter about his feelings for you, and Peter was denying everything that Derek was saying, telling him that was impossible for him to find a woman he would fall in love with.

"It's also beyond rare, I mean we're talking one in a..."

That's when he saw you standing in a different part of the loft talking to Stiles. That's also when he knew. He was in love with you.