I think it is because people picture themselves (sometimes unconscious) as the ones needing help and the mere thought of not being helped incites them to help those people.
Also, helping people gives the satisfactory feeling of having... helped someone. Most of us have been conditioned since birth to build things, and not destroy them. This is also true for helping people, rather than break them.
In some cases, helping people may even bring real 'profit'. Not necessarily material gain, but this can also be respect. You will be respected by those people, and in return, they might help you when you will be in need of help.
Of course, I do not need to remind you that there ARE selfish and uncaring people walking about this world.