Possible version:

I would like to be a volunteer in my spare time. My reasons are as follows:

First of all, it is a good virtue to help others, even in modern society. We all need others’ help in case we get into trouble. Secondly, we are able to better ourselves by helping others because we gain valuable experience and build up our confidence while serving others. What’s more, volunteering makes us feel better about ourselves, thus making our life more enjoyable. More importantly, we can make the world a better place to live in by volunteering to improve it. 

In summary, volunteering can make a difference to our life, so I think it is very rewarding to be a volunteer.