Selfish is hardly the word one would find in the same sentence as the word volunteer but it deserves some sort of answer.
It's quite true that people volunteer or even donate just to feel good about themselves but to what extent? I doubt that people just volunteer for that reason alone.
I honestly think it's how you received that feeling i.e. was it a direct effect or indirect effect.
Clearly a direct effect is when you do it for your own benefit while indirect effect is, I suppose, you putting other people's feelings or benefits before yours and getting these benefits (feeling good about oneself) through seeing other people being happy.
I might sound that I'm just rambling at this point but I needed a space to ponder about this.