You know how Charlie Brown's Snoopy, sitting atop his dog house with a typewriter, always starts his stories "It was a dark and stormy night"? Well, he may as well be Emily Brontë, because that sort of mysterious atmosphere sets the tone of the novel. As a Gothic novel, Wuthering Heights is persistently dark and eerie. There is not a very wide range of tones: it's either grim or grimmer.
The attitudes of our narrators help shape the tone as the drama unfolds, so Lockwood's initial curiosity and fascination convey a lighter feeling than after he realizes how sinister Heathcliff is.
Because one thing is for sure: whenever Heathcliff is around, the tone tends to grows darker. After all, dude says things like this regularly:
Two words would comprehend my future—death and hell:
existence, after losing her, would be hell. Yet I was a fool to fancy
for a moment that she valued Edgar Linton's attachment more than mine.
If he loved with all the powers of his puny being, he couldn't love as
much in eighty years as I could in a day. (14.23)
Oof. We need a SAD lamp—the darkness of Heathcliff's psyche had depleted all of our Vitamin D.
And it's not like the other characters are chipper. You can tell that Nelly Dean really enjoys storytelling, so she tries to sustain a tone of suspense and mystery—that way she keeps Lockwood's, and by extension the reader's, attention. This is what keeps Lockwood up late... and what keeps us reading.