We see James as pretty staid, even after his time at Oxford with John Thorpe. My guess is that on his arrival at Oxford, James was as naive as Catherine was on arrival in Bath. Most of the really social arrivals had found each other, ignored the shy James, and avoided Thorpe for obvious reason. SO Thorpe latched onto James, who was happy to be included and probably impressed by Thorpe's liveliness and "high style" (or is that "high talk"?).

James, being trusting much as Catherine is, had still not seen through Thorpe when he was invited to the Thorpe home after Christmas. Once James met Isabella, all sense of perspective was lost to James.

In a sense, that would mean that James originally fell in with Thorpe for some of the same reasons Catherine fell in with Isabella: acceptance, society, supposed sophistication.