Eh, it's hard to tell exactly because there's so much variance between brains, and their development at any given age.
The general trend of expanding information about it points towards or simply being about impulse control combined with inexperience.
The part of the brain that says "don't do that" grows and develops gradually. No way around that. But it's never "done". There's always going to be some degree of limitation to it, no matter how old you get.
When the person also has less experience with processing a given type of event, exerting conscious control over responses is harder because it's so damn new when you're young.
If you add in the hormonal fluctuations that eventually kick off, fatigue, added stressors, a sense of support for the emotions felt, etc, it's not surprising that even adults occasionally throw a tantrum. The more we feel like there can be a fair resolution, that we have some influence over events and their resolution, the easier it is to control our actions, no matter what we feel, but we still feel things.
I don't know you at all, so it's impossible to give any specific guesses as to what balance of things is inside you that gives you trouble with managing strong emotion. Even if I did know you, I'm not qualified to give a serious attempt at doing so. All I feel comfortable saying in that regard is that the people I've known that have experienced that kind of issue and were not neurodivergent to a degree that could count as a disorder, tend to have a background where their development was irregular.
Big events, consistent small interferences, anything from major injuries or family deaths all the way down to just having siblings that were a pain in the ass to a degree that they never had a chance to learn how to come down from stress. But something made them have delays in learning how to internally cope with their emotions. Sometimes it involves other aspects too.