Well that kinda depends on your wider views. If they consider the UK getting a good deal to be potentially greatly increasing the chances of breaking up the EU (and that this is the biggest thing they need to avoid) then you're right it might. But that's not necessarily the case, and I think that people who point at percentages of trade etc are missing the point slightly too. Obviously the UK represents a much smaller total proportion of trade (both imports and exports) to the EU than the other way around, but it doesn't in absolute terms nor in terms of the number of people negatively affected by new trade barriers. If such barriers cost UK businesses X billion and it costs EU businesses roughly the same, that's roughly the same number of people whose quality of life will be getting worse and the EU has an obligation to avoid that as much as they can. It's a matter of choosing which is worse which, for them I guess, will come down to how likely they think a favourable deal will make anti-EU sentiment grow.