Just speaking personally here, I think being tickled is a kind of pain much as itching is a kind of pain, and the reaction is not joy or happiness at all. The reaction is involuntary, and it tends to look like fun, but I do not think it is fun. I think tickling kids until they’re helpless with laughter is just plain cruel.
As for laughing until you cry, there’s the physiological part and there is also the emotional part. Emotionally it is a very intense experience to laugh that hard, and some people react to intense experiences with tears, as we see during every Olympics.
When I laugh until I cry, I am not really crying at all—it is not physically or emotionally like crying—but it is a kind of physical and emotional stress, and the systems that process both responses are very closely related, or so it seems to me, so one can spill over into the other.
When I really get onto a laughing jag, as I occasionally do, it ends up hurting and I want it to stop. Like being tickled, it can be an experience that feels a lot like pain.