Volunteering...

A volunteer is someone who serves in a community or for the benefit of natural environment primarily because they choose to do so. Many serve through a non-profit organization – sometimes referred to as formal volunteering, but a significant number also serve less formally, either individually or as part of a group.
By definition, a volunteer worker does not get paid or receive compensation for services rendered

Each person's motivations will be unique:

Some people argue that there are no purely altruistic volunteers – altruism is a common motivation but never the only motivation for sustained commitment to serve – they say there is always some aspect of personal gain or satisfaction. Other people disagree - some people argue that there are purely altruistic volunteers since there is no way to prove otherwise.
"serving community because doing service makes ones own life better" — is perhaps the most significant motivation for volunteering. It is often mixed with a good dose of altruism. Included here would be the benefits people get from being with other people, staying active, and above all having a sense of the value of ourselves in society that may not be as clear in other areas of life.
many people have in some way benefited from the work of an organization, or more generally, and volunteer to give back.
Some see participation in community as a responsibility that comes with citizenship.
Many faiths hold that service to others or the community is a spiritual duty or a means of reaching higher spiritual states.
Volunteering is a good way to meet a lot of different people from other walks of life and it is often easy to make new friends.

(in Wikipedia)