At the time of the communist revolution in Cuba, Castro nationalized all the American property in Cuba (not counting the Guantanamo Bay naval base), which is to say, he seized it for the Cuban government and did not pay for it. The US is holding a grudge about this. Aside from that, Cuba has been hostile toward the US throughout the Cold War period, and always opposed anything the US was trying to do. Relations between the two countries are very bad.