How fortunate that we have entire organizations dedicated to explaining exactly that! Rather than flood with propaganda, I'll leave you with this rather concise summary from one of the world's leading experts in intellectual property: http://www.michaelgeist.ca/2015/06/why-canadians-have-good-reason-to-be-wary-of-the-tpp/
There, nop you can probably ignore the rest of this post.
Really short version: at a minimum, Canadians would end up paying significantly more for medicine, and any laws we put in place to protect our environment, or retain control of our resources would be struck down by corporations.
Outside of medicine and IP, the big one that does it for me is fresh water. Without fresh water, we're fucked. Any of us. All of us. Everywhere on the planet. Already, too many of Canada's fresh water reserves are polluted beyond the ability to use for human consumption. The toxins spread into those waters bio-accumulate, making the reasonably significant portion of our population that still rely on hunting and fishing unable to maintain their traditional lifestyle. That might not matter to some, but it matters to me.
Beyond the lovey-dovey feels portion, however, is that fact that even though Canada is one of the most water-rich nations in the world, we suck hard core at managing it. MY home province of Alberta is already seeing subsidence and other nasty effects of our severe drain on underground aquifers.
Right now, today, that maybe doesn't matter so much; we have spectacular amounts of water coming down out of the mountains, and we can divert rivers and other fun things. The problem is that even 10 years from now, the amount of water coming down off those mountains will be a lot< less. The glaciers are shrinking, most of them are almost gone.
Canada's potable and accessible freshwater reserves are something that I believe we need to manage. We need to manage it in order to provider for our own people. We need to manage it because of the raw economic reality that it fresh water is the oil of the 21st century.
In my opinion we simply can't enter into agreements that would treat access to our fresh water as something the government isn't allowed to regulate, tax, or otherwise get a piece of. We can't let a bunch of Americans waltz into our country, pollute and/or export all our fresh water, and stick us with the bill.
I don't give a rat's ASCII if it means we're left out of some oogly-boogly trade bloc. In 30 years, over half the world is going to be begging for access to our water, and we need to maintain the ability to milk them for every last rotten cent. Saudi Arabia created a financial empire on the back of oil, and we need to retain the ability to do the same on the back of water.
It's not like we have the manpower or military to otherwise defend ourselves. Controlling access to our resources and what's left of our environment is the only card we have to play.
And I, for one, am 110% against handing that to the Americans. Not for a trade deal. Not for anything.