The greatest lesson I have ever learned comes from my mother.
And that great lesson is: never be afraid to stand up to someone. If you allow people to walk all over you (especially in the business world) then you won’t get far with your goals or career. If you don’t stand up to some people then they will continue to think that they are superior to you and will take whatever they want.
There is a certain time and place when you should not be so aggressive and give a little leeway (if that person is a top executive or your boss) but that leeway should never make you feel humiliated or insignificant.