When a donor gives to a charity, are you extracting a limited resource? Are you annoying her, making her life worse? The research says no.
Read this commentary in the Wall Street Journal: Money Buys Happiness by Arthur C. Brooks (subscription required). (Full disclosure: the author is my brother.) Note this:
The evidence is unambiguous that donating money (and time) is one of the best ways to buy happiness. People who donate to charity are 40% more likely to say they are "very happy" than non-donors. Psychologists have even tested whether charity makes people happy using randomized, controlled experiments.... The results are clear: Givers of charity earn substantial mental and physical health rewards, even more than do the recipients of charity -- empirical evidence that it is indeed more blessed to give than to receive.
If you're a fundraiser, keep that in mind. You're not taking away from your donors. You're giving them great things. Keep up the good work!