Amadeo's rule, AFAIK was problematic on both sides. He wasn't cut out to be king of Spain (and the Spaniards didn't exactly like him), and he didn't so much want to be king IIRC.
He is said to have said that the happiest day of his reign was the day he boarded the train to take him out of Spain. Also, I think he knew he was a last ditch candidate - since the throne ad already been offered to the Portuguese king-consort, Fernando II, several Bavarian princes and most famously Leopold of Hohenzollern-Sigmaringen whose wife, a Portuguese infanta it was said, was simply dying to be queen. The spate of refusals prompted someone to remark: "trying to find a Catholic prince in Europe [to accept the throne] is like trying to find an atheist in Heaven".
Also, Amadeo showed a lack of tact towards the Spaniards when he was driving through Madrid, and someone pointed out the house of Miguel Cervantes, arguably one of Spain's most famous writers, and he asked: "Well, if he is so famous and thinks he is better than us, we shall pay him a visit".