Anyone who doesn't know why they want to be President of the United States, should not run for President. It is too big of a job to have someone who hasn't put much if any thought into why they are running, or why they would be the best person for the job. Also anyone who doesn't want to be President, shouldn't run either because of all the work and sacrifices that go into to simply running for President let alone doing the job itself. Sacrifices that the candidate as well as their family make.
I believe Senator Ted Kennedy qualified for both when he was looking at a 1980 presidential bid against President Jimmy Carter who just happened to be from the same party the Democratic Party. He not only didn't know why he wanted to be President of the United States, because he didn't want the job in the first place. He felt some obligation to the Progressive Left in the Democratic Party to run for President against Jimmy Carter who was very vulnerable against the Republican Party for reelection. But also came from the center-left New Democratic wing of the party.