It's a lot to do with the amount of drink in a person, not just the presence of drink. Don't forget there's a lot of people out there who can drink socially, who actually only have one or two to take the edge off, and typically loosen up a little and ARE a bit more fun/socially relaxed.
Drunks are bores, introverts coming out of their shells a wee bit is typically entertaining and informative.