I hate it too, although I do 'get' why the assistants do it. They are told to be friendly, and it passes the time for them if they make conversation.
We used to live quite remotely, and 25 miles from a moderate sized town, and we struggled to get things from the little shop closest to us - 2 miles away. So we would often stock up (especially if the big store in the town had stuff with a third off for example.)
We would buy 6 jars of nescafe coffee and 3 packs of 9 loo rolls, and every time, the checkout operator just HAD to say something. Some smart mouth remark like 'are you opening your own shop?'
They also sometimes start reading the magazine I am buying, and commenting on it. That narks me off because I think 'that's my magazine! Fuck off.' 
We also stock up with wine or beer sometimes, and may buy 10 bottles for 6 weeks. 'Oh you'll turn into an alkie' one woman said to me once. Rude. 
One thing that DOES get my goat, is when I draw a large amount out of the bank, like 3 grand, they ALWAYS ask 'doing anything nice with the money?'
Errrrrm, mind your own business. It's MY money! 