It's similar to opening up a candy bar, shaving off a tiny sliver with an xacto knife, and expecting yourself to be satisified. What you've actually done is go through the process of purchasing the candy bar, opening it up, prepared to eat it, and then shortchanged yourself. Now, how many would have the will power to toss the rest of the bar away without another thought? Most would not. The same thing with allowing yourself a tiny taste of alcohol--it's not nearly enough to feel any effect whatsoever, you won't get buzzed or relaxed, but your body has gone through the drinking process, and the natural tendency is to desire more until satisfied.