Because they value what others think of them more highly than what they think of themselves.

Low Self Esteem, you are told at an early age you will not amount to anything good in life, you are a waste, you cannot hold down a decent job, you are always unemployed, what are you good for, you are lazy and you do not deserve to be here etc. It is a learned behavior, from listening to the same negative things hammered into your brain daily. So you start believing it, and the more you believe, the more discourage you become, the more you begin to feel that you don’t fit into society, and that you do not belong here. Sad, but true!

All the prophets since the world began have declared that they were pilgrims and strangers. Perhaps these people are prophets, and don't know it consciously. I mean a person who has a message from Universal Consciousness for his or her circle of influence.