Why would any president run for reelection?

Some may do it because they really want to change things. This health care debate may cost President Obama a lot of votes and democrats loss many seats in 2010. He was even accused of not listening to the American people since the majority said they weren't for it. However, he knows that medical bills are the number one cause of debt in America, medical bills are the number cause of bankruptcy in America and that all leads to a bad economy. So he risked getting re elected just so he could say he helped people. I know it's hard to believe for many people, but there are people out there who don't care about the power. If he cared about the power he would just do whatever the voters wanted.