Why oh why do people insist on getting drunk, going to a gig and then prompty talking/laughing all the way through?

I went to see Malcolm Middleton last night. He was ace, really really good. I was stood right at the front of a half full venue and the offending couple were right next to me. The couple (guy and gal) were fine for the first half of his set. Anyway, during his encore i'd had enough and leant over and asked them to keep it down. The girl turned around to me and said -

"WHAT?? well get him to play some fucking Arab Strap, it's better than this stuff"

I was pretty gobsmacked at that but chose to ignore it. 5 Minutes later she said "cheers for spoiling OUR night mate"

Her partner looked pretty scared and decided to shut her up (i was with a rather HUGE mate).

I just don't understand people who do this at gigs. Middleton had noticed them laughing and commented on it a couple of times.

Anyone else had a similar experience?