Some are called to it. Some fall into it. Some have never considered it seriously while others have acknowledged it all of their lives and will not act on it. I think that a few would admit to feeling like it would be a selfish pursuit; too indulgent to consider if you have responsibilities like a mortgage and a kid and a tuition payment that rivals the aforementioned mortgage.
Right now, I have six applications on my desk from animal rescue organizations in desperate need of help. Each one of them is unique -- yet, the urgency is the same. Their facilities are on the verge of collapse or closure. And, without the buildings -- what happens to the animals they rescue and save?