I think it is because people picture themselves 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.
and Well one reason is in the name of self interest. There are exceptions to this for example, some people just like helping other just because they like being helpful. Then there are people who do it for fun-because they like sorting through other people's "dirty laundry" or helping them with a task of some kind. I'd say at least 63% of the time, it's out of self-interest. 17% of the time, it's out of good nature, and the rest of the time it's because some people just like helping others out.