Depends on your personality I guess! I talk about all the positives I get from not drinking, focusing not on why I quit but more on why I'm sober. People respond to that.
Your particular story though reminds me of a friend of mine who also used to be a big drinker and life of the party type. He didn't quit drinking, but he scaled way way back... rarely drinks now. And anytime someone starts ragging on him about how much more 'fun' he used to be he just laughs and says "dude, I grew up." End of story.
Anyway ... I think the key is just to keep it light. When people ask they're usually kind of checking in. The underlying question is, "Are you OK? Do you mind that I'm drinking? Can we still hang out?" As long as you're friendly and happy, they don't really mind what the answer is.