I think we get to see Roxanne as a developed character through those scenes with Lamar, but when did Barnabas get a chance to?
Roxanne is established as unusually independent, and sees through hypocritres and stuffed shirts pretty easily. She's very verbal about all this, and we do get a chance to know her. Barnabas is never there during those exchanges, though...
Barnabas's life doesn't exactly establish sense or logic or communication as ways of dealing with situations, or people. His life has been filled with magic. It figures, then, that he'd believe in love at first sight, fate, whatever....