I do not like this kind of bet. A bet that no money nor anything is involved. I mean nothing is involved because one of you can decide not to fulfill the betting promise. Example is if your friend did not lose more weight and still decide not to pay for the meal? This can happen. I do not bet with people. I prefer to go to casino or bookie to bet instead.
Agreed, there really is nothing to lose in most cases as someone will usually welch on the bet. It's a great feel of self accomplishment if you achieve the goal, but most people fail with 0 consequences.
Betting money assures that consequences(money) is won or lost.