Why do people tend to avoid desperate people?
From a personal standpoint I would avoid these people because I expect people to be continually getting such needs met. I would wonder why a person doesn't already have a support network in place and be suspicious of them.

I the situation with the beggar I would think this would link more closely to the bystander effect whereby as an individual you feel less inclined and less responsible as there are many other people in the vicinity that could also make a difference.