This answer is an 'unqualified opinion' as I'm not a psychologist. I'd say it is a conditioned thing brought about firstly by the environment one was brought up in. If the parents and guardians were forever talking about money scarcity, using phrases like, "Money doesn't grow on trees you know." and was forever stressing lack and poverty because there was 'never enough money' (There never is, by the way, even with multi-millionaires who have this sort of mindset)
It isn't easy to get out of such thinking. However, it can be done. First you risk giving just a teensy-weensy bit away to, say, a charity or charities, expecting nothing in return. If this doesn't hurt in the longer term, you up it to a larger amount. After sometime you'll realize that you're no worse off financially for giving some of your money away. And before a few years will have passed you mind set of poverty, lack, scarcity would have become more realistic to your real situation.