We all start out as optimists. We believe the other kid will give us a turn on the slide. Our first friends are made without doing any background checks. Trust is what we're born with, but we develop skepticism from life's disappointments.
For some it takes only one let down. One betrayal, and immediately the walls go up. For others it may take heartache after heartache until the hard outer shell develops. The more we are disappointed by others, the less likely we are to let people in without proving themselves.
It's hard to argue the logic of being cautious. After all, it's inevitable that someone will let you down. But if our walls continue to grow higher throughout life, one day they may become unscalable. Be sure to every once in a while lower the walls, even if slightly, so others have a chance.