Who want's to be the most powerful person in the world anyway?

I know I wouldn't want to be the most powerful person on earth, waaay too much responsibility/stress!

I'll assume you're talking about the U.S. Presidency, since this is in the politics section.

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Well, Obama has described how he came about wanting to run for President. It all started from his family life as a child, that made him want to make a difference in his community. So, he became a community organizer, helping people who needed it get the things they needed from the legislation.

Then he realized that he could only have so much of an effect on law and stuff from that end, since the best he could hope for were things like amending laws, never creating new ones, so he decided to go back to school. He got a law degree from Harvard, and instead of taking it to Wall Street where he could have easily lived richly (Harvard Law graduate for crying out loud), he went back to Chicago to work as a legislator, then an Illinois Senator, then a U.S. Senator.

His first year in the Senate, he came to kind of a similar conclusion as before: he was doing good where he was, but he wanted to do even better. So, he decided to run for President, so that he could organize the greatest community of all: the country.

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By contrast, this is why McCain is running for President, as it says in his own memoir:

"I didn't decide to run for president to start a national crusade for the political reforms I believed in or to run a campaign as if it were some grand act of patriotism. In truth, I wanted to be president because it had become my ambition to be president. I was sixty-two years old when I made the decision and I thought it was my one shot at the prize." --John McCain, "Worth the Fighting For: A Memoir (2002)

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I guess everyone's got their reasons. Some people seem to be following a great calling to leave this world better than they found it, others seem to feel like they're entitled to the power it brings.