I think, these are the in the genes of the some people that they always wanted to help others.
There are some people who soft hearted and always ready to help those in need.
There might be peoples who started helping other as they themselves have suffered.
Then they are some people who are inspired/ motivated by other people to help other.
The people who are in such charity work have believe that god will reward them for such good deeds.
They are always will to contribute towards the society to make this world a better place.
Involving in such activities give them mantel peace and inner satisfaction.