How do I know when to say, "it's not my responsibility"?

I have saved 7 kittens this year. I found homes for all of them, took them to the vet, gave them medicine, etc. Today my wife found a baby squirrel at the school where she works. I picked it up from her school. I called around town for about an hour until I found someone to help me. I then drove 30 minutes to this wonderful woman’s house, and I gave her $100 dollars as a donation to her cause. And I was late to work because of it.

This is not self-aggrandizement. This is a serious question.

Why is it that whenever I see an animal in need, I feel like it’s my responsibility to help that animal? I feel as though the universe put the animal in my path for a reason, and thus I must do something about it. If I walk away, I feel sooo guilty. It’s like this is my own personal sin. If I added it all together, I’ve probably spent close to $1000 dollars this year providing for animals that are not my own. The money isn’t really the issue. (Even though my wife and I live on a new teacher’s salary, and my meager pittance.) My in-laws (for instance) look at my wife and I like we are insane.

I guess I’m just feeling overwhelmed by all of this at the moment. Help!