This is the central thing that changes in a person with age. When you are born, the only thing you do is pick up new things. Literally nothing else. When you're young, picking up new things is how you improve your social position. It's what you do to even be talked to in the first place. It's what you do to get a girl/boyfriend, or be the best student in class, or to be the best (or worst even) employee at your first job ...

Once you have a good social position, or at least one you're happy with, you stop doing this, and you grow ever more irritated at others doing it ... because it's your social position that they're coming after. And they're younger, more motivated and hungrier. More than that, a decent chunk of these people want a better social position, even if that means taking yours.