Why did the presidents of the United States want to be presidents?

Many believed their ideologies were the best for the country. Some just wanted power, but look at it with modern presidents. Obama, while I do not agree with him, believes his Health Care Reform is best for the country, and he became President because he wanted what was best for his country. In order to better his country, he needed the power to correct what was wrong in his eyes. Donald Trump was talking about running for a while because of the economy, being the big business-man that he is.