Pretty sure it has to do with him aging. At some point in your life you realize your own mortality and start to decide what really matters in your life.

Even in my 30's, I've consciously started to make decisions about how I want to spend my time. Do I want to play video games for 3 hours, or go out and mountain bike for 90 minutes, or spend time with my dogs or my family? I'm pretty sure Bill went through the same thing, albeit he has a ton more money and those decisions were easy for him to use his resources to try and be a huge agent of change.

It probably also has to do with his legacy. Does he want people to remember him as the tyrant who ruled MS with an iron fist and his idea to rule the world? Or as a kind, gentle billionaire pouring his vast resources into solving some of humanities toughest problems?