Another possible reason for Gatsby's not wishing to drink too much is that since liquor is known for loosening the tongue, Jay Gatsby may worry that his fabrications about himself may inadvertently come out. For, even when he makes conscious efforts in his tales about himself, he sometimes slips and tells the truth. For example, he professes to have traveled many places, yet he thinks San Francisco is in the Midwest.
Playing the role of the Oxford man is no easy task for Jay Gatsby; if he drinks, the challenge will be even greater. Besides this problem, Jay Gatsby perceives himself as the pursurer of "the grail," and "the dream." As such, he is loyal to the dream and acts politely in order to ingratiate himself with the wealthy.