Not staying true to what you want can set you up for a very self-destructive pattern.
Being nice often leads to doing things — or not doing things — for the sake of everyone else's comfort level. You hold yourself back in an effort to make other people happy.
You say "yes" or "maybe" when you really want to say "no."
Even when it's something little like going out to a bar when you really just want to sleep, not staying true to what you want can set you up for a very self-destructive pattern. Pretty soon your life is no longer your own, and you've forgotten exactly how to get what you want.
You have to stand up for yourself to be happy. You have to do things because you want to, not just because someone else wants you to.
Of course, this isn't an excuse to get you out of chores or taking your mom to the airport. It's life, and you're going to have to do things you really don't want to do sometimes.
But if it's a task or situation that makes you feel unsafe or uncomfortable (or if it just sounds really boring and no one you love is benefiting from it), say no. Always, always say no.
Life is way too short to not put yourself first. And I don't mean that in a selfish my-way-or-the-highway kind of attitude. It's about never letting other people come before your goals, dreams, and priorities. It's about not letting people tell you that you don't deserve something you want.
Because you do.