What if I like being "too nice"?

I've always been what many people consider "too nice" - will back down from an argument/concede to end the fight, will keep quiet and go with the flow if the majority want to do x and I'd rather do y, I never raise my voice, and have rarely if ever gotten truly angry about something. To me it is just not sweating the small stuff. However, everyone in my life - parents, friends, wife - keep telling me I need to learn to get into more arguments, disagree, put up "healthy" boundaries etc. to avoid being walked over. The trouble with this? Every. Single. Time I have tried this - multiple different situations, with many different people in those groups - always end up in the same situation; a huge argument where my natural demeanour means that I "lose" the argument as I am yelled at until I give in. Now isn't this "standing up for yourself" thing meant to make people respect you more and yell at you less because they see you won't just "take their sh*t" all the time? Because so far all that has changed is that I am in far more arguments and fights, pushing people away - and not even reaping the benefits of this increased respect I'm meant to be getting.

In light of all of this I have come to the conclusion that I actually preferred it when I was "too nice". My life was actually better because no arguments. Yet people keep telling me to keep going with it (it's been nearly a year now). I prefer being "too nice". Why is that a bad thing?