I never understood why Sauron wanted to take over middle Earth and rule over the lesser beings who lived there. There's a line in A Fire Upon The Deep saying "they had better uses for their time than exterminating cockroaches in the wild". Same with Sauron and hobbits.

 

Have you ever met a power-hungry manager in a fast food store? They hold so tight to power over others because it fulfills them. Sauron is the ultimate bad manager.

 

Cause all the Ainur are fascinated by Elves and Men.

Before the Children of Eru, it was just flora and fauna everywhere. The children are sentient.

You wonder why someone would want to interact and rule over that then just... what? Ainur are immortal, they literally forged the planet into what it is. Anything worth doing they have done, aside from watching these new kids on the block with their free thought.