People fear being taken advantage of by others.

Sometimes I am reluctant about giving public aid because I am afraid that the person may be dangerous or have bad intentions behind their petitions. Because the people who ask for my help are strangers, I don't have any information that gives me some security. Most of the time when I am asked for money or favors on the streets I am prevented, because they could be lying and taking advantage of my help.
However, depending on the context I could do an exception and give public help more relaxed. For example if I am in a safe public place with security guards and the people in there have a good appearance.Other situation will be an emergency where I am aware that something bad is happening and others need my help.
Even though there is always the possibility that the petitioner takes advantage of the situation, there are some cases when you take the risk because in one way or another you feel secure. However, besides this exceptions, prevention is better than the cure.