I reply thusly:
"A life of what? Dealing with all the crap you deal with? Going to a soul-crushing 9-5 day job in some tiny cubical withering away while contributing nothing major to society? Sure I work in a dead-end retail job, but at least I'm happy. At least I love my life. At least I enjoy it. When you contribute something meaningful to humanity, when you actually help people, when you actually do something that matters, then come back and lecture me. And even then, it wouldn't make you better than me, it would make you an asshole. No wplease shut the hell up and get away from me."
It changes a bit from person to person but its the same basic thing all 'round.
Worked well so far.