It could be a couple of things:
He honestly lost interest: Now, this can be because you somehow embarrassed or offended him.
He's trying to hide his interest; maybe he feels like he's been embarrassing himself by showing too much interest.
I saw my crush the other day, but I didn't go up to talk to her. For one, I feel like I've been showing too much interest in her, and when I do, the girls run, so I scaled way back. Two, I wasn't mad at her, I just didn't want to be to obvious; Three: I also feel a little embarrassed because I don't know what to say to her because I'm really shy around her, and when I do have a conversation, it feels boring and embarrassing. 4) She's shown no real sign of interest, and I don't want to become more interested and feel really hurt when she turns me down and starts ignoring me. 5) I'll be seeing her again really soon, so I don't want to make her feel like I'm a stalker or creep, because I'm not. 6) It was a bad week, too, and I didn't want to feel worse by embarrassing myself or feeling more rejected.
He's deep in thought, and / or stressed; think of it this way, the gears may be winding around so fast that it's tough to concentrate on you.