It does seem to me that the Morland family is rather sheltered, and I would imagine it was because they had laid back, easy going, but busy parents who were raising 8 other children in addition to James and Catherine. As Ch. 1 states about Mrs. Morland: "Mrs. Morland was a very good woman, and wished to see her children everything they ought to be; but her time was so much occupied in lying–in and teaching the little ones, that her elder daughters were inevitably left to shift for themselves."

I could imagine that also could be similar to Mr. Morland and time for his sons as well. For sheltered people who also have very forthright parents who say "Catherine almost looks pretty", flattery by others goes a long way! If you love parents who say you almost look pretty, you're going to definitely attach yourself to someone who actual compliments you (at the right times, of course), particularly by wanting to spend time with one! So far, I don't particularly care for the Thorpes, but I see them more like I see Mrs. Allen, etc - a bit self-absorbed and self-interested. No fatal flaws in them per se. For James to have spent a whole week with them and still like the family is still saying something.