Look at any popular religion and you will find a heavy emphasis on sacrifice, selflessness and delayed gratifications (that helps strengthen our control over our base desires). This is because early civilisations (that used a religio-political construct to form societies and nations) recognised that a healthy society only flourishes when humans are taught to fight their innate desire to be selfish. You can observe this when kids demand attention as they become jealous of their siblings or play with toys with anyone - they have to be taught to share, and appreciate it as a value.

Your perspective on the in-group and out-group is relevant of course - it isn't healthy if one is self-sacrificing without addressing their own needs. (However the degree of that varies between eastern and western culture. Eastern cultures, which have more civilisational history, perhaps don't necessarily see as it in such rigid terms?). Moreover, my point was that having wealth tends to affects your perceptions to "expand the size of the in-group" - money does provide more avenues to be independent, and thus a financially independent youth can (ignorantly) miss opportunities to learn how to have healthy co-dependent relations as most erroneously assume that such relations are not important to them because they have achieved "independence". Learning to be independent is important. But then learning to understand the importance of co-dependency is another step to become a mature adult.