Maybe it's a particular kind of balsamic vinegar, or maybe it's the only kind of cat litter your feline will deign to use. The point is, when it's not there — and you've been looking forward to it for days, and you've set aside some precious time to shop, and you're not sure when you'll be able to squeeze in another trip to the grocery store, and you're stressed by all the rude shoppers banging around in shopping carts and/or yelling at their kids to stop harassing them for toxic-looking sugar products — it's really hard not to button-hole a stocking clerk to give him a piece of your mind.
When this urge rears its ugly head, just try to recall two things:
One, just because the stocking clerk is on the front lines, clearly placed there as minimum-wage cannon fodder for disgruntled clients, that doesn't mean you should release all your pent-up rage about humanity and the tyranny of the material world at him. The target is too easy.
Secondly, you catch more flies with honey. This is a hard one to remember, but it's really true. An ingratiating smile and a few soft words of encouragement to the manager to order more stock will have a far happier result than a hissy fit in aisle three.