Several generalizations:

• If you're making a cocktail it's not much more work to make two at once.
• People who know a lot about cocktails like to share their knowledge.
• People who are newer to cocktails aren't sure what to get.
• Shared experiences are fun.

What if a cocktail bar had a policy that when someone orders a drink another person could ask for a copy, and both people would get a discount? Come up with catchier phrasing than "I want one too!," advertise it, your bar gets a differentiator, and everyone wins?

(This came out of a lunchtime discussion at work, though seeing as I don't drink and have spent very little time in bars so my sense of what people would like isn't very strong.)

Update 2015-06-10: This would be illegal in MA, however, as it involves charging "a price less than the price regularly charged for such drinks".