Cathy's love for Edgar Linton is summed up in this conversation between her and Ellen (in Chapter 9):
'Why do you love him, Miss Cathy?'
'Nonsense, I do - that's sufficient.'
'By no means; you must say why?'
'Well, because he is handsome, and pleasant to be with.'
'Bad!' was my commentary.
'And because he is young and cheerful.'
'And because he loves me.'
'Indifferent, coming there.'
'And he will be rich, and I shall like to be the greatest woman of the neighbourhood, and I shall be proud of having such a husband.'
In other words, it is (like most love) not 100% "pure"; yes, there is love, but there is also the convenience of money, honor, and pride.
By contrast, Cathy has little to gain by her love for Heathcliff. Although he eventually amasses some money and property, she is in love with him long before that. There would be no honor in marrying him, as he is a foundling with no family pedigree. It is just that there is a blind, overwhelming passion between Cathy and Heathcliff. This passion haunts them throughout their lives, and continues even after their death, when their ghosts are seen together on the moor.