[Silhouette]

Well both parties start out a negotiation not knowing what the other sides limits are in certain areas, having society not interfere allows you to not give away this information to the other side.

Unfortunately, having society not interfere may also mean that you do not have that information yourself, even if you think you do.

If the negotiators are acting in the interest of their own people then you could assume that they will know quite well how far they can go.

Right, but that's a big "if". International trade agreements are like a textbook example of the principal-agent problem. It is extremely common for the negotiators to be adopting positions heavily influenced by special interest groups and business communities, with little if any regard for the effects on the actual people of the country those negotiators ostensibly represent. In the case of the EU, that issue is magnified further, because the EU itself can be (and often has been) used in a similar way.

[DominikR]

There is no doubt in my mind that the EU negotiated this in secret because they knew that society would be opposed. They likely would have tried to ratify this treaty afterwards in a stealthy way by suppressing media coverage or "encouraging" media give it a positive spin.

But I cannot prove this, that's why I'm also listing a few reasons for secret negotiations that could be acceptable for those who are inclined to give the EU the benefit of the doubt.