The saying goes that nice guys finish last and for the early part of my career, this used to really bug me. I was only good at being nice and my conscience would usually stop me from doing mean things at the frequency that I thought might be required to get ahead. With the passing of time it is now clear that the saying is not true. I have seen a few not so nice guys come and go. On balance the vast majority did not get ahead and many matured a little and snapped out of being mean because it wasn’t helping their cause.
The truth in the saying is that some assertiveness certainly helps. Many nice guys lack assertiveness and many mean guys have it in spades, so maybe this is where the saying originates.