Why is he really nice and happy to talk to me and then tries to avoid me?

This has been going on for a while. He is a bit older, we see each other outside quite often and we never used to talk just watch each other. Until finally we started greeting eachother, laughing, talking, asking things and getting along. I was so sure he likes me, his eyes seemed interested in me for a year. His face lights up when I talk to him and he really pays attention to what I say but he has his guard up and wont flirt or be playful with me. If i try to be playful he gives me a funny look his eyes get bigger and he tries to change the subject (and I'm an introvert i don't flirt) anyway some days he's seems to really like me and others he keeps his distance, acts anxious avoids me won't even look at me and even seems a bit angry and wont even talk to me, like a vicious cycle. I asked him if I was bothering him and he said "no of course not don't be silly." So why all the anxiousness, avoiding me, one day nice one day rude, one day very interested next avoiding, one day open one day closed. Long question but I would appreciate some advice.