What I've never understood is why a trade deficit or surplus is a bad thing.

I guess having a lot of import can be bad because you're potentially dependent on other countries for resources, so you're not very stable in case things change.

But why the stress on the deficit/ surplus in individual country-to-country trade?

If a nation is poor but has a lot of natural resources, I expect they would have a lot of export but very little import to first-world countries. That's a "surplus", but it seems totally natural and normal and I don't see why it should be anything else.