We like positive surprises and and fear negative ones.
That means(suprisingly) that it's better to have a consistently negative experience than to confront on that's sometimes negative and sometimes neutral.
On the other hand, a positive experience that's positive all the time pales in comparison with the experience that's sometimes neutral but often nice. Beyond a baseline of goodness, you're better off awarding a few people a random discount or a bump up in priority than you are making something consistently (and boringly) slightly more pleasant.
