Karl Friston's free energy principle is probably roughly 80% of my reasons to think they're coupled. The rest comes from studying integrated information theories, architecture of brains and nervous systems and neutral nets, more broadly information theory, and a long tail of other scientific concepts (particle physics, chemistry, biology, evolution, emergence, etc...)

It's hardly an unreasonable supposition: the one definitely conscious entities we know of are also the apex intelligence of the planet.

To put it another way: lots of things are conscious, but humans are definitely the most conscious beings on Earth.