More it's just the way I've pretty much always been. Before I was even really aware of it, I apparently figured out that I couldn't control the outside world but I could control how I reacted to it, so that was what I focused on. One could sort of say that I did it simply because it made sense to me, but even that makes it sound more conscious than it was. It's more that it just never occurred to me to do things any other way.
It was only much later that I discovered that there was a philosophy called "stoicism" that advocated that.