He looks away when we talk sometimes - what does it mean?

so I like this guy at work but I know he has a girlfriend so I respect that but he's been friendlier lately.

he jokes around with me sometimes and I make fun of him kind of teasing him. today he said something about my diet and he was saying it's not healthy or whatever and I jokingly said 'so?' but when I look at him, he looks away for a bit when I look into his eyes. what does that mean? I know I have a small crush on the guy but I haven't flirted with him because he's got a girl. what does the look mean? before we talk too and then he said good night.