Cathy's feelings for Heathcliff can be summed up best by using her own words.
"he's more myself than I am"
Cathy goes on father than this by saying.
"My love for Heathcliff resembles the eternal rocks beneath -- a source of little visable delight, but neccessary. Nelly I am Heathcliff"
Cathy views her love for Heathcliff as a neccessity. She feels that she and Heathcliff are the same person.
Cathy's feelings for Edgar are very different to what she feels for Heathcliff. For Edgar she feels no great passion as she does for Heathcliff. The reasons she gives for loving Edgar are because he's handsome, pleasant to be with, young, cheerful and because he loves her.
Cathy herself describes her love for Edgar as being like 'the foliage in the woods.' She admits that time will change the way she feels about him.
The main reasons she gives for agreeing to marry edgar are, because by marrying him she will be 'the greatest woman of the neighbourhood' and the fact that he is rich.
The reason she gives for not marrying Heathcliff is that it would degrade her to marry him, because Hindley has reduced him to the level of a servant.
"if the wicked man (Hindley) in there had not brought Heathcliff so low, I should not have thought of it."
The other reason she gives is because she believes if she and Heathcliff married then they would be beggars.
She believes that by marrying Edgar, she can use his money to get Heathcliff in the position she belives he belongs in.
"If I (Cathy) marry Linton, I can aid Heathcliff to rise and place him out of my brother's power"
Cathy really does believe that her marrying Edgar will end up helping Heathcliff.