Hello, Yang! I can think of a only couple expressions about volunteering.
1. To do one's part, "I volunteer at the anmal shelter once a week because I care about animals and I believe I should do my part."
2. To give back: "My father had a car repair business, and he often taught poor teenagers how to work on cars, because he believed in giving back to the community."
That is the best I can do...maybe someone else can do better. My own father used to quote a saying from Winston Churchill, who is very famous but I don't know how famous this saying is:
“We make a living by what we get, but we make a life by what we give.”