Who's Reading My Gmail?

Everytime I open a letter on my gmail site I get a list of advertisers on the page.
Every advertiser relates in some way to words within my letter. If I write that my back hurts and I'd like to get a massage - voila'! - pain management centers, chiropractors, massage therapists, or any sellers of services relating to that text, no matter how vaguely, will appear. Wow. (Don't want to use too many exclamation marks. I heard it was a girlie thing and who'd want to be accused of that?) I digress.
So, wow, not only is Google reading my mail but they're selling it to others. It's like having all these people looking over my shoulder. They're not just looking over my shoulder, they're invading my private space. I used to think of my private space as the area within two feet of my physical being. But now I think about it as the space inside my head, where I keep my most private musings. What are all these "stuff sellers" doing there? And how free is this email service really?
This leads to the really creepy thoughts about who's out there spying on me? You may be concerned about the NSA, FBI, CIA or even Mossad but I wonder how GM or GE or Pfizer will use this info to talk me into buying things I don't need - to separate me from my hard earned dollers. Or maybe the newest political party will use my thoughts to prey on me, my fears and concerns.
Do I have to go back to snail mail? Does Verizon record my telephone calls and sort through my words? To what end I couldn't guess.
I'm starting to get a little nuts now. Maybe I'll have to stop communicating all together.
Uh, oh. No talking? No writing? Is that like no water?
If I think about it, I have to guess that someone has probably been reading my email since the start. So, I'll continue doing what I've been doing for so long - spilling my guts on the electronic page but maybe not all my guts.