Generally I respect the choice of the person who asked the question, but if they don't select an answer after a day or so I sometimes do, particularly when one in particular seems deserving. Every now and then, some of us, including me, do act a bit more impatiently.
Not everyone approves of over-hasty selections, or awarding of stars when there is only one answer.
There is no mechanism that stops any person from selecting an answer even after a best answer has been chosen. If you select a different answer, the one who got it first will lose their star. If you simply don't approve of the best answer selection, you can remove it by clicking on the star, even without selecting a new best answer.
Edit: When I began this answer, another answer had already been awarded a star. By the time I finished, the star had disappeared (it wasn't me who removed it!).
