The American Constitution, a poorly-designed relic of the 18th century whose every imitator eventually fell apart, is designed to force compromise. Every two years the whole House of Representatives, and one-third of the Senate, is up for reelection — but the president is elected for four years. As a result, it is extremely common for one political party to control one or both branches of Congress while another controls the presidency. So unless it's one of the rare moments of one-party control, parties have to come to some sort of compromise on key political questions to keep the basic functions of government rolling along.