Sweden and Denmark have fought somewhere between 15 and 20 wars with each other, more than any two nations in the world.

/u/vonadler did a summary of them a year ago. This is counting only from the time they can be said to be unified countries. There were other conflicts before that too, I've seen estimates well over 30 if you include the viking age but for the purposes of this discussion, let's stick to unified countries.

The explanation is pretty straight-forward, Sweden and Denmark are really old countries, we've had 1000 years to fight each other.