It’s Obama’s old toothpaste-out-of-the-tube trick, on a grand scale. Once all the countries have agreed to it, what are you gonna do? Go back and have to renegotiate every point with every country? Not feasible. So show it to us first, then to the other countries. Who’s boss here anyway?
Well, that's kind of the point. The theory behind free trade is that it's a benefit to all the parties. It isn't like we're just doing a favor for the other 11 countries by letting them trade with us.
No country, including the US, is going to make the hard compromises they must until they know what they're getting in return. We would be asking them to make politically risky moves knowing that congress would accept their concessions but amend the deal to take ours out.
This agreement has been under negotiation since 2005, with the US joining in 2008. Multiple administrations have been involved with lots of input from congress, business, labor, etc. It remains to be seen how good or bad the TPP will be, but it should be debated on it's merits, not the process/secrecy red herring.