Means to not believe this should be your standard, basically consider it a gift.
Been used in a bunch of movies, etc...
As horses develop they grow more teeth and their existing teeth begin to change shape and project further forward. Determining a horse's age from its teeth is a specialist task, but it can be done. This incidentally is also the source of another teeth/age related phrase - long in the tooth.
The advice given in the 'don't look...' proverb is: when receiving a gift be grateful for what it is; don't imply you wished for more by assessing its value.