>I don't understand why Americans continue believing that democracy is the only way for every population in the world

Well, ideology. I believe my way is the only way for every population in the world too, and I fight for it to happen. Of course, each place adapts to their own condition, but I believe my core ideology is the way for humanity as a whole, and I believe it is the same for people who defend western american-style democracy.

What part of "defending western american-style democracy" involves imposing it on other countries and being mad when they don't adopt it?