You were looking too intently and didn't notice how he looked back.

Scenario:

You and Gregory have known each other since elementary school. Every year you start to notice that he is becoming more attractive, and you don't even know why it suddenly caught your attention. When you were alone with him again, you took a close look at his appearance, taking advantage of the fact that he was still talking about himself. But you didn't immediately notice how his gaze met yours, and he fell silent for a while.