it's not wholly about gain
Certainly not. They're going to do what they think is best. Probably this is why I get called a cynic, but I would wager they also have ulterior motives which play a large part. Desire for fame/power, wanting to make an impact on the world...they are doing it for themselves for others, not because their one and only goal is to make the world go round.
It's like when someone donates money. Yes, they're doing it to help. They also are doing it because it makes them feel like a better person. The only time I wouldn't question that they're doing it because they believe it's the right thing to do is if they never told anyone unless asked about it, and weren't expecting to be asked about it. Which would likely mean that no one would ever know they had donated something. Likewise, they also wouldn't tell anyone that they had done so not to prove to themselves that they aren't doing it for another reason, but because it does not matter if their action is known.
Of course, when was the last time anyone with common sense ran for president?
Probably this election. I didn't pay attention to -every- person who tried to run. They just didn't get very far because they weren't known/weren't apart of one of the two parties.