Being president is not about changing a country for the better. It's about a lot of things. You need to keep people happy. In a fantasy world, it seems that doing good deeds will make your people happy, but that's for King Arthur. Real life is a cruel world. People won't be satisfied so easily, and you have to make lots of rough decisions that will hurt people. You have to choose if you'll satisfy the wealthy or the poor. Strengthen our education or make it so everyone can get a degree. All decisions have pros and cons.
If you truly have a kind heart and want to help the world, then start something on your own and go out and help those who are in need. Always remember that money is nothing and never fall to it's temptation and do everything you can to help the ones who have less than you. Society has forgotten how to care for those who need it, so it's best to only look at yourself and what you can do for those around you.
Power was never meant to rule people, it was meant to serve them.
The only thing you'll accomplish nowadays by being president is a rich puppet for greedy corporations.