I found as I aged, my anger/ resentment turned outwards rather than inwards. I was pretty shy when I was young and, being forced to be around others- as we all are, I think I just absorbed all the hate and feelings of inadequacy. I also believe I grew up with a (suspected) narcissist so- the hate towards me was extremely intense there.
I guess we put up with all the shit when we're young because we don't have quite the experience or capacity to view the whole picture. Plus, we're brought up not to question authority and we know we aren't in the position to do anything about it.
Also, anyone coming up against a (suspected) narcissist- especially that young, it can feel like being randomly punched in the gut. It's so shocking that a person can even behave like that. It defies logic so, if we aren't strong enough to blame them, we likely will start to accept blame ourselves question our own sanity.
I guess becoming an adult, we are given slightly more freedom to hold our own opinions. I think also, when we reach the stage we realise we are deeply unhappy, we start to look around to figure out why. So, we start to question how much is in fact our fault. What wrongs were done to us? How fair/ unfair the whole system is. At that point, I think we can probably find a fair bit to be resentful for.
My Dad once read a note back I had written in a book I gifted him when I was in my early teens maybe. It was full of apology for being needy. I was basically apologising for existing. The worst thing is, I expect he probably prefered me like that! Hearing that now though- it just makes me think- wasn't it obvious there was something very wrong? Wasn't it concerning that your own child was feeling guilty about simply existing and needing you?
I can't say I enjoy feeling angry though. It's over very basic stuff I can't even change a lot of the time. I just have an antinatilist angst that I'm alive in the first place and expected to earn the 'privelage'.