Over the past few years I have come to the conclusion it is how we are wired to some extent but also how we were raised.
If I look back at my family, both sets of grandparents volunteered, Scouts for 40+ years on one side, both of them - its how they met, the other side, Lions, Church, Playgroup, Village events, you name it they were involved. My parents, mainly my mum was on any committee going, PTA, Save the Children, multiple church groups, Carnival Committee, Choir treasurer etc. So for me volunteering is normal and part of life.
However I have asked others about this - when we get an email from the PTA I spend days agonising about how I can help and trying to rearrange commitments to make it work, others just read it, think that doesn't work and delete and move on never to think of it again!
As a volunteer who needs to recruit others to save units from closing, I just wish people would say they aren't interested or its not their thing. Listing all the reasons why not can irritate when I am stood there thinking - yep so do I, ie young family, busy job, etc etc.