Why is he always staring at my lips?

So my crush looks at me a lot in class. And we have all 7 classes together & sit next to each other in some. But sometimes we'll just be staring at each other from across the room, Actually we've done it soo much that now it's kinda of a usual habit that if that doesn't happen something's wrong like one of us must be upset or something. So anyways, I've realized that a lot of the times when he staring at me, he usually is staring into my eyes but lately he's been looking at my eyes then lips then eyes. He even has just straight up stared at my lips whether I'm talking or not. Why does he do this?
Why is he always staring at my lips?
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