Everyone who is diagnosed with cancer must wonder how they got it.
I think about it often. You want to know what you did because then you can stop doing that, so you can get better.
People usually blame:

genetics
something you ate
something you didn’t eat
something you did
something you were exposed to

People have also mentioned these causes, though not as often:

something someone did to you
something you thought
Something from your past life
god

You may never know for sure why you got cancer. Generally people will think you are crazy if you tell them why you think you got cancer. (Unless, of course, you say you got it from something like smoking).
At risk of sounding insane. Here’s my current theory of how I got breast cancer:

My cell phone
Lack of vitamin D

I kept my cell phone in my left breast pocket- something I did, and something I was exposed to. And I avoided sunshine. I didn’t drink milk with vitamin D added – something I didn’t do and didn’t eat. Now I take 3000 iu’s of vitamin D per day, try to get sun, and keep the phone away from my body as much as possible. Plus, I may have a genetic disadvantage.  In the meantime, I will continue to do  as many things  as  I can to survive.

Here’s an article from Feb 2010 GQ on cell phone hazards
and an article from Science Daily.