Very easily for two simple facts:

1. It isn't a zero-sum game

2. Canada and the EU aren't the only players in the world.  Those not involved with the trade deal can lose out.

For point 1 the idea is that companies can focus on what they do best and can expand to bigger markets rather than expanding to a wider range of products that they might not do as well.

For the second point consider the example someone (or some company) in Germany is looking to buy a product and comparing a Canadian and Japanese company that make comparable products.  Right now both companies face import tariffs, but with free trade between Canada and the EU the Canadian company would no longer face those tariffs and be at a competitive advantage.  So the Canadian company wins out, the German individual or company win out through lower costs but the Japanese company loses out.

Conversely if Canada does not sign a trade deal and Japan (or the U.S.) does then Canadian companies lose out.