How is the spread of democracy justifiable?

I'm not trying to create a political uproar here, but I'm just curious -- how can we justify the spread of democracy? I mean America is not a democracy, which means unlimited majority rule. America is a constitutional republic. I guess I think of the shoe on the other foot -- what if communism or some other sort of dictatorship government like they have in the Middle East was forced upon us? We'd be pretty pissed too, eh? (Now, I do realize our quality of life in the USA is much better than many other countries, especially in the Middle East...) I'm asking with complete naivete here -- how is it justifiable?