Hiya
You can and will drive yourself nuts asking Why. We all do that, whether the cancer is small or extensive. The answer is that there isn't often an answer. You have a family history of cancer, I have none. No-one.But that makes no difference at all to either you or me. It doesn't make it go away, and I don't believe knowing why will help really. I wonderede too if I had done something to cause it - but I eventually realised that knowing that wouldn't help me now. So I stopped trying to firgure it out.
The only day you can do anything with is today. What happened yesterday is gone, we can't change it. And we have no idea what MIGHT happen tomorrow. There's no point in ruining today worrying about either. Carry on life as if you have forever left. DO all the things you want to do. Get all the treatment you can have to help you, and then just try (yes, I know it's hard) to shift it out of your thinking brain. Don't waste a precious moment.
I hope you have successful treatment, I hope you have many years of life to live. I hope you can be as happy as you need to be.
much love
magicmum