I get upset when my boyfriend is too friendly with other people.
What's wrong with him being friendly?
Because I feel like he is hitting on them by being so friendly.
Being friendly is a good thing.  You shouldn't be jealous of other people.
He said he would never cheat, but I can't help but feel worried.
It sounds like there's no reason to suspect that is cheating or planning on cheating on you other than him being friendly. Being friendly is a good quality though and not a sign that someone is going to cheat on a partner. You should try to be most trusting of him so you don't drive him away.