Mr. Lockwood infers that Heathcliff hides his true emotions. By not telling us directly, Bronte foreshadows later events and creates suspense.
The book begins long after the events of the main story. When Mr. Lockwood meets Heathcliff, he finds the man, his house, and everything and everyone in it coarse and unwelcoming.
Lockwood also does not get along well with people, and so he infers that Heathcliff is the type of person to keep his emotions close to the vest.
I know, by instinct, his reserve springs from an aversion to showy displays of feeling—of mutual kindliness. He'll love and hate equally under cover, and esteem it a species of impertinence to be loved or hated again. (ch 1, p. 5)
Heathcliff finally admits it when Lockwood complains about his dogs.
Guests are so exceedingly rare in this house that I and my dogs, I am willing to own, hardly know how to receive them. (ch 1, p. 6)
Lockwood makes all kinds of social blunders at the Heathcliff household. He finds Mrs. Heathcliff alarmingly rude, but it turns out she is not the hosts wife. She married Heathcliff’s son, he tells Lockwood. Lockwood then assumes that the young man eating with them is Heathcliff’s son, but he isn’t. He is Hareton Earnshaw, and he has a temper.
Poor Lockwood can’t make heads or tail of the situation, and it gets worse when he goes wandering out in a snowstorm and can’t find the way back.
By not telling us what really happened, Bronte establishes a lot of predictions in the reader’s mind. Clearly, there is something going on here other than just being antisocial. The strange characters and events foreshadow trouble. Something happened in the past that made them this way, and we are about to learn the story. The reader really wants to know what happened, and is therefore increasingly curious. This creates suspense.