What are some reasons why a guy would act this way?

So i have this 19 year old co worker that i actually get along pretty well with. Our encounters usually consist of being playfully mean and pulling pranks on each other so its always laughs or play fighting with us. (Yes, I can be immature for my age. It is what it is.)

But then i notice when i try to have a normal conversation with him or when im trying to actually say something nice, he gets all weird and shy i guess. He cant maintain eye contact, he cant stand still, and one time he even just walked off before he even finished his sentence. It was just...bizarre to me. I feel like ive said something wrong.

Any ideas what this can mean? Is it just an insecure young person thing?