Why is he acting this way?

I have ben friends with this man for years. He even once told me he loved me and I said the same back. I haven't seen him for a year. I am still not well, but we see each other every day regularly through work. Sometimes he is really nice to me, then when we are in the canteen, he acts aloof and cold, like he doesn't know me. If he sees me in the corridor I notice he puts his head down and looks away. I can't interpret the expression on his face. If we were fighting it would be appropriate, but we are not, other times he is quite kind and caring for me. Why is he acting in such a way? I am finding it hurtful. With all other women, ALL other, whether he is attracted to them or not (and I know this cause he has told me in the past who he likes and that) he is flirty confident and always friendly.