Maturity is linked to words like evolving and changing or gaining wisdom and a broader perspective about things. Oftentimes what we considered ideal or important no longer is or we want something different. We're either growing together or growing apart.
However people mature and evolve at their own pace and sometimes some come to realize they are running out of time to live out their "dreams". When it's all said and done: Life is a (personal) journey.
People also naturally gravitate towards "like minded" people. It's not unusual when friends and partners start to desire or have less and less in common they start to drift apart.
Some folks may call it a "midlife crisis" or a "midlife awakening". Either way these people decide the use what time they have left to make the most out of their lives.
The whole notion of having "bucket lists" is founded on this principle.
Living for yourself isn't "selfish". It is after all (your) life to live.
The world may not owe you anything but (you) owe yourself the world! (You are responsible for your own happiness!)