] Perhaps the thought of a man being interested in her has just never entered her head. She doesn't expect it, so she doesn't see it.

I can see that. :-) This fits in with the way in which she's been brought up. A girl who delights in hearing, "Catherine looks almost pretty today", from her parents, isn't likely to jump to conclusions about the young men she knows in any case.

In the case of John Thorpe in particular, they have been pretty much thrown together because of her friendship with his sister and his friendship with her brother. Since she sees the relationship only in this light, it simply doesn't occur to her that he would see it differently.

Funny, as I was writing the above, I started to say that JT and Catherine were also thrown together by the romance of their respective siblings. I realized as I was typing that she doesn't even see that until Isabella tells her of the engagement, does she? Could that be further proof that she doesn't see romance unless she's actively looking for it? She seems to see a hint of it with Henry Tilney and herself, or at least to hope that this is what she sees.

] Or it could be that the thought of a relationship with JT is so horrible that her mind won't let her proccess it :)