It could be private interests. In the example you gave us, you say that a person who runs for prime minister will do so with good intentions, to improve the state of the country. In that case, we agree.
Is there an example of someone who entered politics, not necessarily to improve the state of the country, but rather to ensure that the people they did business with didn't lose money? That person would be in a good position to help them, but it would put the country in a difficult situation.
Do our laws protect us from situations like that?