You can be nice, just make sure you think about what you're actually doing before doing it.
Letting a car go in front in the situation above: you're probably causing an accident.
Letting a car go in heavy traffic when there's one lane each way and everyone's stopped already anyway: won't cost you much time and you've allowed that person to move on with their life instead of being permanently stuck at an intersection he's never going to be able to get out of unless someone yields.
I live close to a few intersections where if no one is nice and yields, it's impossible to join unless you barge your way in and hope people stop. But to be fair, these aren't designed like death traps like the one above.