This section really impressed me. You have little, almost irrelevant details that make it a joy to read, and I could so believe that Sam would make his point using these analogies. To me it works wonderfully well as a dialogue driven story, because that's how the characters are - words are important to them.

I loved the little word play in Josh's mind: Or maybe that should have been a pretty, lousy friend, Josh thought, and the way he flirts with the waitress.

A couple of typos spotted:
"Sam, you could give me a lift back." should probably be "Sam, could you give me a lift back?" and You check you workings should be You check your workings.