Let's say you get a reaction it usually means one of the following:
It likely means the guy is possessive/clingy.
It likely means he can't control his emotions.
It likely means he's not confident in himself.
It likely means he's going to be mistrusting of you later.
So why do so many (ok many may be a sweeping generalization but I've noticed it more often than not) girls do this. If a guy is getting jealous it's not a sign he likes you, or cares for you. It's a sign that he's likekly on the controlling end of the spectrum or not well adjusted to be honest, yet it's often mistaken that this means he's some gentleman strolling in on a white horse.