Regarding the waiter's choleric disposition toward Larry, I think he's just a little off-kilter and does not like LD from the get-go, without more reason than that Larry just rubs him the wrong way.
However, with regards to the acupuncturist's weird look to Larry, I may be wrong, but it has always been my understanding that this was the smug, satisfied look of victory, following the fruition of his master plan.
He was confident, given the little he knew about our hero, that if he were to send flowers to Larry after their session that Larry would be confused and put-off by the gesture, and may or may not interpret it as some "Japanese gay thing". But either way, he would not be comfortable returning for a follow-up visit. The doctor strategically has his receptionist solicit a reason for cancellation when she calls Larry to confirm his next appointment, and he tells her he's "better", thereby executing the final stage of the acupuncturist's design, and securing him the five thousand dollar prize.