When in the checkout line, if you have like 20 items, and the person behind you has two or three, will you let them in front of you? I have done it a few times, but not many, and try not to make a habit out of it.
If you say, "Yes, of course!" then where do you draw the line? What if 30 seconds later, another person with only one item gets in line behind you. Will you let them pass you, too?