"quieter, more subdued personalities are often misunderstood in public situations." (quoting W.D) Maybe he doesn't feel free to be friendly to everyone.
He "...spent the rest of the evening in walking about the room,..." This could mean he just observed his friend's neighbours and he couldn't interact.
However his "sincerity" about Lizzie's beauty was very offensive and innapropriate. But I don't know...He said what he thought..unpleasant, but, personally, I think I could live with that..