I am a firm believer in wallowing. As I've said before, I think we get whacked out when we try to make ourselves avoid what we're feeling and bottle it. Therefore, I condone wallowing. However, it is possible to let one's self reside in the world of wallowing for too long... this can produce either obesity, or depression. Wallowing does not have to be a road to depression; it is, however, allowing one's self to say "yes, I feel like crap" and act like it. The sign that you need to wallow can be identified quite easily. If you are tired of everyone saying "it will all be ok" or "you'll get over it" or "think about the bright side!", you need to camp out on the self-pity campground and cry a little.... consume some high-calorie foods... and basically feel sorry for yourself.
When wallowing, it is best to have someone, such as a best friend or sister (it's rather convenient if the person is both), so that you know you are not completely alone in the world, AND so that the person may supervise the wallowing as to prevent it from morphing into a deep depression. It is even better if the accompanying wallower is an individual who is experienced in the field of wallowing, and comes bearing your favorite kind of ice cream and potato chip.
If sadness was not an emotion, we would not be capable of feeling it. Throw it a party.