Jealousy is about being insecure with your own self thinking that you are not good enough to keep the person you care about more interested in you than in someone else they might encounter. So it's a bad thing not to have confidence in your own abilities to keep the person in your life who liked you for who you are in the first place. By being jealous you change others opinions of you. It tells them that you have no self esteem, it changes your happy self to a suspicious, questioning self and it ruins your ability to enjoy a happy time with your loved person because instead you have to worry about any one else that might me around that would catch her eye.
it all depends on what or who you are jealous of. if you are jealous about something simple, like your friend got a new dog and you don't have one, but want one, then it's perfectly fine to feel that way. everyone likes dogs, for the most part. but if you are jealous of someone because they have the most expensive "toys", like super expensive cars or a giant mansion all to themselves, then you shouldn't be jealous. be happy with what you have. rich people tend to be snobs about it anyway. lol
It's not bad it's natural. It shows that you care for that person. But don't over due it!!! Be interested in what's going on but don't be pushy. That will only make things worse. I would die if my gf cheated on me but I trust her and I have to give her space to be herself. And if it happens then she's not the one for me