Why would a man ignore the woman he likes?

Today for example, I was talking to my friend. He is a college teacher and there is this woman that he has liked for about a year, I know she likes him too since one time I heard her talking on the phone with her friend about how much she likes my friend. Anyway, when I asked him why he ignores her when she smiles to him or when she makes the effort of getting close to him, he gave me no answer. I asked him if she had done or say something bad or if he had heard something about her, he said no but he could not explain his reasons to ignore her 😧 He has made it clear many times how much he likes her and really admires this woman but instead he decides to ignore her. That led me to this question, why would a guy ignore a girl he likes?
Why would a man ignore the woman he likes?
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