The reason why the Romans change their government from a monarchy to a republic?

They did not, it happened the other war round, Rome started out as a republic ruled by the senate, then after Julius Caesar (who refused to be king/emperor when this was offered to him), Rome became a monarchy ruled by an emperor.

Because they were "uncomfortable" with monarchy. That is my opinion, though. Hope that helps :)

Actually Rome had just less than seven kings when it was born, then it became a republic until its end. They had consoles, senators and democracy but it was an empire. Rome wasnt governed by a king or emperor per se, but by senators although the emperor had more powers than nowaddays presidents.