When the limiting resource is money it's quite clear that we should prioritize the uses where it goes the farthest. If there are three organizations that can distribute antimalarial nets for $5/each, $50/each, and $500/each we should just give to the first one. Similarly, if I have $5 I could use it to have my electricity be generated by wind or I could use it to fund distribution of an additional antimalarial net. I can't spend that $5 on both, so I have to choose, and I choose based on which I think will do more good with the money.
When the limiting resource is happiness, however, prioritization comes less naturally. I could stop taking warm showers, take the bus instead of driving, spend less to donate more, go vegan, donate a kidney, not run fans in summer, or do any of a very large number of things to make the world better at some cost to me. The more I do, the better, but the less happy I am. If I chose options without looking at how they trade off my happiness against benefit to others it would be like choosing what clothes to buy based on how much I would enjoy wearing them and not considering how much they cost.
I also posted this on my blog