Because you want him to pine for you, dream of you, die for you, while you're safe as the lovely Mrs. Edgar Linton.
7 seconds sound clip from the Wuthering Heights (1939)) classic radio play.
You can hear this line at 00:36:56 in the radio play.
Quote context
[...]
- Tell him Heathcliff asked me to marry him. And that I've said yes. You hear? Yes!
- You can't. He's not a man. He's something horrible and dark to live with.
- I know why you say these things. Because you love him.
- Yes, you do love him. You're mad with pain and jealousy at the thought of my marrying him.
- Sit down, Cathy. I won't say I'm not surprised to see you.
- Heathcliff, is it true? Did you ask Isabella to marry you?
- Oh, you mustn't do this villainous thing.
- And that's what I intend to do.
[...]