- you are motivated
- you choose to do things without being told
- you want to make something better
- you are willing to help
- you can do more than you have to.
There are probably more reasons, but those are some that I give.
I ran across a piece on giving, showing others believe in this advice as well. Giving is a positive exercise, and while you will feel good, it shows that you are someone that is less likely to take more out of job/life than you put back. You will likely be an asset more than you are a burden.
Think about giving at some point in your life. It might not be now, but I do believe that you should give back to others when you can. At work, however, you ought to think about doing this quarterly, helping others when you can.