How come nice guys/gals/small furry creatures from Alpha Centauri get dumped for jerks/bimbos/hyperfungal Rigellian psuedoshoggoths?
Mostly for the same reasons jerks get dumped for nice guys, bimbos get dumped for other bimbos, and hyperfungal Rigellian pseudoshoggoths get rejected in favor of lesser Altairian shaggy-toothed carno-weeds. No matter how much more keenly you feel it when it happens to you than when it happens to someone else, the fact of the matter is that getting dumped/rejected/passed over in favor of others happens to pretty much everybody, no matter how nice or mean they happen to be. Remember, too, that your perception of someone you’ve just been dumped for is unlikely to match that of the person who just dumped you: most people tend to view their rivals through a jaundiced eye to begin with; your dumper’s tastes, turn-ons, and values are rarely quite the same as yours; and it’s likely that your rival will feel jealous of _you_, which tends to get in the way of your rival showing you his or her best side. It’s also worth noting that the person who dumped you may be trying to make you feel better
