Some people volunteer to gain career experience, to build their resume, develop skills or learn new ones. In addition volunteer work enables others to make contacts with people in their chosen career.
Some people volunteer to build self-esteem, to make themselves feel better. To feel needed, to feel that they have a place in this world - to feel like they are making a contribution to society.
Some people volunteer for the social aspect, to climb the social ladder while helping those less fortunate.
Some people volunteer as a result of the instilled set of values they have a set of values that compel them to act on deeply held beliefs about the importance of helping others.
Some people volunteer because they have felt the cruel sting that life may have afflicted them or a loved one with. For example someone might volunteer for the American Cancer Society because a family member died of cancer.
And some people do it because they want a better understanding about different people or the organisation they serve, "bridging the gap," so to speak, in order to better understand their community and themselves