There is a cliché that nice guys finish last. This seems to be built on the premise that nice guys are doormats to other people and perhaps even attract people who will abuse their inclination to please. Although I can see how those conditions could be true, I do not think it has to be that way.
Nice doesn't have to mean passive or weak or prone to being taken advantage of. Nice can still be assertive and confident and willing to require fairness, because nice isn't merely agreeableness. It can instead mean to have sensitivity for another's desires while politely insisting on the same consideration.