Why do countries with multiiparty systems often have coalition governments?

Multiparty means that the political system has several groups writing laws and running the government. The only way to get anything done is to have a coalition and compromise because without that the people's business doesn't get done. We have a good example in our own government of what happens when political parties stalemate and block doing the needed work. Literally nothing gets done and the citizenry suffers from the partisan political process.