One’s “own best interest” can take a lot of different forms. Especially when the number and variety of plausible candidates are finite. Your preferred candidate for a given office will rarely line up perfectly with your own values. There’s a compromise there.

If I vote for my own finances, it may come at the cost of my morals. It I vote for my own moral interest, it may cost me more. If I vote for my own power, it may cost someone else their freedoms. How heavily do I weight my own interests against those of a wider society? Political identities and philosophies are complicated, and can’t necessarily be reduced to a single binary choice that is “best” in every scenario.