Signs a guy does not like you and why?

there's this guy I flirt with at work. I try to talk to him through text as much regarding work and I also try to flirt with him a bit by messaging late at night (once), for example. He doesn't initiate conversation with me. But he does talk to me in person - but he always has this 'lusty' look on his face or voice. I recently tried to contact him but he seemed annoyed and that hurt me a lot. Im very shy in person and dont talk to him. So i have these questions going on and on in my mind and I really want to forget about him but I guess I just need closure first.

1. Does he like me?
2. Should I back off a bit or just be direct and ask in person why he's being rude? What else can I do?
2. What are signs a guy is generally not interested?
3. Why would a guy not be interested? (physical attraction/if the girl is too shy, etc.)
Signs a guy does not like you and why?
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