Well, looking at it from a logical standpoint (oh boy, here I go XD)
Tickling is both a pleasure and a pain response at the same time. Your nervous system isn't 100% sure what to make of it. It actually triggers a few different "sensory" receptors in your skin, including the ones that feel Pain, soft or hard objects (depends on the methond of tickling) and in some cases hot and cold (depending on the intensity of the tickling). It isn't normally enough to hurt, so the pain receptors send, literally, half of a message.
The brain, in turn, interprets this jumble of partial signals the best it can. For some people, like many of us here, it's a pleasurable torture we can't get enough of. I myself don't usually laugh but I will wiggle around and giggle or purr (yes I know, not very manly of me is it? For some reason the few girls that have tickled me have found it strangely attractive/erotic *shrugs*) and if my g/f is the one doing the tickling it can be sexually stimulating. How pain receptors can trigger such a response is beyond the understanding of medical science. It is much the same as being "suffocated" can increase sexual pleasure. A good example is when you hang somebody- if their neck doesn't snap and they asphixiate to death, their body responds by becoming sexually active (men become erect, women have their vaginal tissues swell and become moist). A most peculiar response to imminent demise I must say.
This stimulation (be it platonic or sexual) causes a few changes in the body. All encompassing is an increased heart and respiration rate accompanied by an increased metabolic rate. As the tickling continues, your body quickens every activity used to sustain itself. You require more oxygen and energy to support the effort of breathing heavier and for your heart to beat faster and harder. You're sweat glands become active as your temperature rises from the effort and, if you are struggling against it (bound or just trying to hold still even) all this is increased as your major muscle groups expand and contract.
All of this results in your body burning calories. Just half an hour of roughouse ticklefighting can burn, I believe it was, almost 100 calories. Due to the stress on the body, an hour burns nearly 300 calories. Should you carry it out for two hours, you can burn in excess of 600 calories, not including your bodies "cool down" phase afterward.
Beyond the physical aspect, tickling can be very healthy emotionally and psychologically. Everyone knows that laughing is very good for your health. In a lighter, more gentle tickling that elicts gentle flowing laughter, your breathing deepens and your heartrate rises slightly. You're muscles relax and you momentarily forget any stresses you have had. Laughing loosens up the vocal chords as well, provided you don't go overboard. I personally like pulling sweet flowing giggles and small yelps of laughter out of my victims. I don't like when it almost seems as though they are in pain because it can have negative effects on the body at that point. I can say from the (sadly) all too few times I have been tickled for more than a few seconds, it is VERY relaxing to just lay there and let someone you trust have at it, knowing they won't abuse the power you have given them and that, should they do just that, you can always get them back.
Psychologicaly, tickling is a powerful destressor. It releases endorphines in the brain that stimulate a feeling of wellness and pleasantness. If used as a prelude to sexual actions, tickling can not only stimulate the body but prolong it's "agony" as you build towards climax. When used as a softer, more sensual feeling, tickling helps to increase the bond of trust between two or more people which, in turn, reduces the stress of being around others.
*smiles a little*
I am not a scientist of any sort. In fact, I am only 19 years of age myself. However, since I can remember I have been fascinated with the human body and have always enjoyed tickling young women my age. Watching their reactions gives me the pleasure of not only tickling someone, but of also making someone else laugh and feel, if only a little, better.