It's totally OK to feel & express anger when we're kids... Because there are so many things in life that we don't understand, that we can't agree with, that make us disappointed.. When we're kids, we don't know yet that everything has a beginning & an end. We believe that if something breaks, it can't be fixed & nothing can replace it. If we injure ourselves, we don't know that naturally (most of the time) the wounds heal by themselves. We can't see how we create unpleasant situations ourselves, we don't see the way out of misery. We think that our expectations have to be fulfilled & that others, especially our parents, should be our servants forever. And of course until a certain age we're fully dependent on the adults around us, and they can't always understand what exactly we need - this is often irritating.
Generally, when we're kids, our world view is rather limited. So we often get disappointed & angry.
When we grow up, on the other hand, with time we learn more about how the world works & a lot of things stop upsetting & angering us... But because when we were kids, most of us weren't raised in an optimal way - we were not taught to accept our anger & disappointment - we were taught to dismiss them & to bury them deep down.. We were taught to suppress ourselves instead of learning to give space to all our expressions - we were taught to dismiss parts of ourselves, to become scattered, not whole. (Of course, how would it happen differently.. our parents themselves had no idea about any of this. They did their best.)
Ideally, we needed to learn to give space to our expressions (e-motions = energy motions), experience them fully then and learn to accept life as it is (we can't change the past & the present, for they've already happened - we need to learn from the past & present & make new decisions for our future). We need to learn to not judge our expressions & our learning process.
Most of us (almost all of us) never learnt this as kids.
So there's lots of unexpressed anger, disappointment & disagreement that got stuck inside of us. These expressions of ours are a great mechanism for processing life when we're kids (and btw when we're kids, it doesn't harm the world in any way). But when we grow up & have learnt a lot about the world, why do we still experience these expressions?
Well, it's because they still need to be expressed. So our inner-children constantly look for an opportunity to be seen & accepted as they needed to be a long time ago.. It's not the 'adults-us' that feel this anger & disappointment, it's the 'kids-us' that still need recognition, acceptance & love.
So I conclude that this emotional roller-coaster is totally immature. It stems from expectations, lack of understanding of the world order (ignorance), desire to control the people around and strong attachment to the people around.
A mature person accepts everything & gives space to everything first and then makes decisions based on their values & life goals.
But before we can become mature, we need to give space to our ignored inner-children.