As a summary we can say that the friendship between our nations is a modern thing, despite the millennia old relations, which actually didn't went further than any two neighbouring medieval countries' did. But certain circumstances in those times denied real enmity from formation.
Between our countries a formidable and well defined border stretched, the Carpathians, which prevented convenient border conflicts, petty raids. Settlements up there were few, far, and small, not worth the effort. Our rare conflicts therefore had different reasons, and different theaters, the Bohemian throne was such a cardinal point for example.
On the other hand we had similar enemies. Both country lay along the east-to-west/west-to-east highway, as obstacle to any expansions from these directions. German hammer fell on us both not on one occasion, and had to fight against various eastern neighbours, most notably had to suffer from Mongol-Tatar invasions, campaigns and raids. Although we had the Carpathians as a nice cushion from the east, while Poland is entirely open, another highway run through our country, from the south, south-east. First Byzantium, then the Ottomans were the anvil when the aforementioned hammer fell, and ofc their roles changed, often had to suffer the punches from the powers of the south, where the Ottomans in the end became a common enemy for us and the Poles.
Tho I have to add, during medieval times certain societal characteristics developed - such as the highest percentage of nobility and it's consequences - which allowed and caused similar mentality to evolve.
So the rivalry eluded us but suffering from similar fate - losing independence, suffering from similar societal backwardness - created mutual sympathy and understanding. Giving mutual assistance was the next step, and the common cause and more importantly the common struggle finished up forming the friendship.

Even today we are walking in the same shoe, have to jump the same obstacles, licking the same arses, eating the same turd sandwiches. We have similar reactions to events, similar interests which influences these reactions. Not many issues we could clash about. We have to kinda watch each other backs too.
Many of our people perhaps don't care, but many other still keeps the sympathy alive.