Equivalently, you shouldn't really be all that happy that a thousand people get something that makes them really happy when your actions reduced the number of people who received whatever it is by 500.

I've got no problem with being happy that a thousand people get a bunch of utility (assuming they are people for whom I have altruistic interest). I would not be glad about the fact that I somehow screwed up (or was unlucky) and prevented even more altruistic goodies but I could be glad (happy) that some action of mine or external cause resulted in the boon for the 1,000.

I have neither the need nor desire to rewire my emotions such that I could unload a can of Skinner on my ass.