I think that the moment Stiles realized Lydia loves him too was when she said “I’m not leaving you” because, in that moment, he heard himself in her words. He remembered all the times he wouldn’t leave Lydia, or didn’t want to leave Lydia (in the Eichen house, at the animal clinic with young Derek, before heading to Mexico for Scott, etc.). He remembered what he felt for her in those times. He remembered the inner, powerful feeling that compelled him to never want to leave her side- especially in times of danger. What Stiles feels- what keeps him at Lydia’s side despite the threat to his life- is what Lydia feels as well. Stiles could hear it in her words and see it in her eyes. Lydia’s “I’m not leaving you” was all the confirmation he needed, and he acted beautifully after finding that out. He didn’t pry the words out of her. He didn’t even hint at needing or wanting to hear her say it. Instead, he was content with the revelation he had on his own and made sure the last thing Lydia heard from him was that SHE was loved. His last request- what he wants Lydia to remember in the wake of his realizing she loves him- is that Lydia remember she’s loved, and that is a divine thing. God bless Stiles’ heart.