He started to say something but didn't, why?

So there's this guy in my math class, and he's been sitting behind me and we've gotten to know each other and we're good friends now. Over the past couple of months I've kinda sorta developed a crush on him. I'm not sure if he feels the same, but he does stuff that makes me suspect he does (touching, staring, constantly teasing me). However, today I was in the lunch line in the cafeteria with one of my other friends (a girl), and we were joking around and he was in front of us in line. Once we'd gotten there, he turned around and looked at me and opened his mouth to say something, but just shook his head and started laughing. I tried to ask him what he was going to say but he just kept saying "It's nothing". He's not one bit shy and this was beyond frustrating. He's usually really open about everything and I want to know what he was going to say. Why do guys do this? Should I be concerned or was it just nothing? Any advice would be awesome.
He started to say something but didn't, why?
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