Horizons are funny things. Plonk them in the centre of the shot and the landscape seems to eat up the entire screen. Put it in the bottom third and the eye draws down through the expanse of sky back down to the land again.
Wherever you stick it, our eyes get drawn as if by gravity to the ground again. It’s almost as if we’re afraid to look up. I dunno.
Oh, and you’ll incur the Wrath of Amateur Photographers Everywhere if your horizon isn’t exactly flat. Personally, it doesn’t bother me much.
Horizons are what you make them, I guess.