One of the reasons it is hard to make friends as an adult is because the associate between sincerity and naivety grows stronger over time. The more you get burned or see people get burned, the more closed off you become to real interpersonal connection. This is the default behavior. Once you realize this though you can do something about it. Ultimately you have to be willing to be vulnerable, to stick your neck out in order to cross that barrier from 'associate' to friend
