Reddit top voted comments....
Sparrows. Yes they ate crops, but they also ate locusts which destroyed crops much much worse
Failure. If you don't experience sitting down and being humbled, you're all the more likely to not only let success go to your head but to take things for granted.
Chemotherapy. Generally speaking, it destroys most cells, but cancer cells typically have a higher metabolic rate and are thus more sensitive to chemo agents.
Sadness. You can't be happy all the time. It isn't healthy. You have to take the good with the bad. You learn to cope and move on from it.
Parking enforcement. I know they're a pain in the ass, but in some cities there would be chaos without them to remind everyone that you can't just park your shit everywhere.
Killing someone to prevent them from killing someone else