No-one is born jealous: it happens for a reason. If you've been brought up in a loving, trusting family with happy parents, you're less likely to be disillusioned about relationships. On the other hand, if one of your parents cheated on the other and they separated, it's only natural that you're more aware of infidelity.
Our experience of relationships is also important: if you've been cheated on, left high and dry for no reason or messed around by the ex from hell, it could quite well lead you to become jealous.
The fear of seeing the same thing happen again can not only make you vulnerable, it can also make you doubt the intentions of anyone who shows interest in you. It can also make you see all men in the same light, which isn't true case and isn't going to help you find the right one!