A guy who likes the girl a lot, easily gets jealous if another guy talks to her and is sequestering her attention.
This is commonly the case with guys who are relatively protective with who they choose to date and are unabashed about showing it.
That personality translates to a girl who isn’t even his girlfriend.
Guys like this are typically used to dating and focusing on one girl at a time.
They put most of their energy into talking to that girl as opposed to talking to multiple girls at the same time.