How do you know if a guy is falling in love with you?

So we are both in serious relationships with other people. He started telling me he is always thinking about me, has been anxious to get to talk to me, and that he misses me. Is this a problem or is he being friendly I really like him a lot but I don’t want to hurt him because It can’t go anywhere.

How do you know if a guy is falling in love with you?
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