coreytothecup,
so why does hakaros hatov only seem to be a chiyuv when something GOOD happens to you? it should also be for when someone does something BAD?
Note the change from “something” to “someone.” I would have agreed with your statement had it read, “it should also be for when something BAD happens to you.”
To put it another way, there are two kinds of hakaras hatov: gratitude to Hashem, and gratitude to creation, e.g., people. We should feel gratitude to Hashem for everything that happens to us, because He is in control of everything and knows what is best for us. This is not the case with people, however. So if someone were to heaven forbid hit me, I think it would be amazing and right to show hakaras hatov to Hashem, who is acting for your best interests, but not to the hitter, who was certainly not acting for your best interests. Otherwise, as miritchka pointed out, you would be expressing gratitude for a sin, which is not appropriate.