Humans always do what's best for them.

This is an entirely false statement.
The true statement is:"People generally do what they THINK is best for them and people are generally idiots."

There is a huge distinction here.
For example:

A vote seller thinks that he is doing what is best for him but the fact is that by acting in such a way he only achieves a short term perception of gain while in reality greatly hurting his investment.
If the time frame is long enough the person will be unable to even realize that fact due to seeing his stake increasing.