Why would a guy get jealous of a girl he isn't even dating?

There is a guy I was talking to (we were just friends and had just met), but he had stopped talking to me for a little bit, during that time, I thought he wasn't going to ever talk to me again. So I decided to move on and have been hanging out with my other guy friend. Then the other guy reached out to me again, a few weeks later and when I had told him i had been spending time with that guy, he got jealous. Why do guys get jealous of a girl they aren't dating, does it always mean he likes her? He said he knows he shouldn't be, but he feels cheated out on. He said he wants to see me but is worried it might be pointless because he is afraid I'll talk to other people if we were to be more than just friends. He said he gets attached really easily and doesn't want to be hurt. He seems to be very afraid of being hurt, but wants something. What are ways I can help this guy overcome his feelings of being afraid and show him I don't want to hurt him?

Why would a guy get jealous of a girl he isn't even dating?
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