Because going through a struggle tends to turn people inward and makes them see everything, including the harm they do to others, as if they were a victim just doing what they have to do to survive. It is very hard to overcome it because you have to become a kind of martyr, who accepts the reality of their suffering but who's too holy to blame it on everyone else, or even on yourself.
Kids are often nicer than adults because their lives are so easy, but if they don't learn to bear a cross (this is the best metaphor I know) they'll become nasty as they get older, as people reject them and their health starts failing. It's one thing to be a nice young man, another one entirely to be a nice overweight balding 60 year old with joint pain and a skin disease. That's not to say it's about age. It's about how good you feel, and age is just the big conveyor belt that everyone goes down whether they're ahead or behind their demographic.
You can watch this play out in you next time someone says or does something annoying when you're still smarting from a minor injury, like a stubbed toe. You'll tend to act as if they were the ones who stubbed it because blame wants to earth itself.
I think that what you call "learning to bear a cross" is really an application of empathy, and an important one. It's the knowledge that "if I have an emotional meltdown, those close to me will be forced in to the role of caring for me, which emotionally drains them."
It's such an important skill to know when to hide or bear your hurt, to spare others the burden of care, and when to share your hurt to acquire some care. It's a difficult balance to strike because if you go neglected for too long, you will completely meltdown, but if you elicit care too often, you will also elicit compassion fatigue from your carers (especially as an adult, I mean, parents will pretty much pour all the care required in to their young children, it's not that they are limitless, but the limits are way higher).
To be less transactional about it: you don't make every negative event be all about you, because you recognise that things affect others, even things that seem to only affect you. That requires a quite refined and well-developed sense of empathy where you are balancing various overlapping and conflicting needs of, potentially, several people at once.
Generally, people who experience childhoods which are impoverished of the stimuli necessary to develop robust senses of self and the coping skills that come with it are left much less resilient to life struggles in adulthood.
If you have developed a sense of self early in childhood, then you are able to develop empathy for others, if you bring empathy in to a relationship, then you can be trusted and trust others. Think about it, you cannot rely on a relationship that does not have trust as it's basis, you cannot be trusted in a relationship if you cannot perceive or understand the other persons needs and feelings, and you cannot perceive or understand the other persons needs and feelings if you cannot perceive or understand your own.
If you have a sense of self, and then you have developed empathy, and then taken that empathy in to relationships, and if you have then made the moral choice with that to be a kind person, then that enables you to form resilient relationships. And you take those relationships with you in to adulthood. These relationships, and the coping mechanisms that got you them, are highly effective cushions for the shocks of adult life. Life changing struggles can still afflict such people and bring them down, but then they are much more able to recover since they still have the scaffolding they developed in childhood and they can rebuild.
For those with a childhood impoverished of the necessary stimuli, it is much more difficult, because they are rebuilding it all from scratch.
And then there are those for whom the experiences of life have been so damaging, possibly combined with genetics, that they may never be able to develop a functioning sense of self, or empathy and go through their entire lives. And yeah, the older you get, the more difficult it gets because you may not have the attributes that would enable you to turn it around earlier in life (family or school friends with whom relationships haven't yet been completely poisoned, good looks enough to meet someone new, meeting ambitious people your age at the start of their careers who are looking to network, etc..)
Kids are often nicer than adults because their lives are so easy, but if they don't learn to bear a cross (this is the best metaphor I know) they'll become nasty as they get older, as people reject them and their health starts failing. It's one thing to be a nice young man, another one entirely to be a nice overweight balding 60 year old with joint pain and a skin disease. That's not to say it's about age. It's about how good you feel, and age is just the big conveyor belt that everyone goes down whether they're ahead or behind their demographic.
You can watch this play out in you next time someone says or does something annoying when you're still smarting from a minor injury, like a stubbed toe. You'll tend to act as if they were the ones who stubbed it because blame wants to earth itself.