People are basically neutral. It is the perception that defines them as good or bad. We subdivide ourselfs into various groups, and develop a feeling of belonging.
Such groups are the generations. Objectively speaking, we are still the same kind, the very same humans.
However, as different people fit into different entities, they begin to see their generation as a version of themselves, thus becoming part of it.
Eventually, it leads to misunderstanding of others. We start to criticize the ones to come, therefore creating an endless cycle of disapproval.
But this is in the very nature of humans. As hard as we try to fit into other peoples' shoes, we simply cannot. We can try to see their world with our eyes.
We - subconsciously or not - neglect little but important details that bring a feeling of selfishness into existence. We can only be as selfless as our minds allow us to be.
Not because we don't want to but because we can't.