Of course you can volunteer, and it actually helps a lot to feel better. I think my point would be that maybe you choose a profession, for example you are an engineer, you can go and have a steady, well paid job at multinational car producent, most of the time you don't do anything that makes any sense, that makes the world a better place. And you can be a intern, or a regular emloyee for a worse salary, at a startup where you contribute to the research of filtrating the COˇ2 out of the atmosphere, and it is not sure that it'll ever work, it isn't a secure position. If you have some talent or knowledge what you worked hard for, and you have to choose between contributing to something that just maybe could someday make the world a better place, or have a steady job somewhere where you have to work with something in what you can't believe at all, it is not that easy. These jobs just kill your 'soul' and rob you from everything what should be important. You can do a lot as a volunteer but you can do more if you choose a job where you can use your unique knowledge to achieve something that helps someway, to someone.
And even if a car somewhere is a necessity, my point would be that young adults tend to 'sell' their ideals, or dreams about making a difference, because of the short term advantages, like having a good salary to buy things that aren't neccesary, in a lot of case cars, the next pair of shoes to wear 4 times, or a 5th watch, a new phone, when the previous is still perfectly working...