First I'll say that it's useless to be dogmatic on this issue one way or the other. If you haven't tried it, don't knock it. If you have tried it, you know what it's like so state your opinion.
Generally speaking, I think moshing is a "reaction"...sort of an "involuntary reaction" to be more specific. One person here touched on it...when the power of the riff overtakes you, you just lose it! I have actually experienced this myself. Where I'm from, the pits are crazier than anywhere I've ever gone to either play or see a show. REALLY rough! So, it's not logical to want to hurl yourself in the midst of this flesh and bone tornado. But the many times I have moshed, it was like...I couldn't help it! And for me, it always happens on the super-heavy grooves of songs...particularly Suffocation's "Funeral Inception" ULTRA HEAVY groove...."GOD FORBID!! GOD FORBIDDEN!!" It's like I'm watching them jam one minute, and the next, I've just come from the pit, and I have no idea what I did or who I may have anhiallated.
I believe that as a general rule, musicians tend to want to watch the bands performance, and the die-hard fans want to mosh. Being a musician, I'd RATHER watch the band (which I do 80% of the time). But there are times when the urge to unleash this inner rage overcomes my desire to watch the musicians. Our band used to have a sort of "cult following" of skins (which was REALLY weird, and there was a show we played where the venue had mirrors on the walls...bad idea! At the end of the show, the mirrors were gone, and when the lights came on it looked like a slaughter house in there...literally! Several people were hospitalized and even more went to jail. Now, this is extreme in any person's book. I don't care how strong the "urge to unleash"...I'm thinking that urge would be curbed considerably the first time I felt a mirror plunge into my head!
I saw the Suffocation (the BEST EVER) and Behemoth show in Atlanta recently. There were actually quite a few moshers for Behemoth, but out of the 4 bands that played, Suffocation got the most moshers by far. And they moshed to mostly their new stuff. Here's why....MOST of these were guys around 20-25 years old. They've probably only heard Suffocation's new album since they were only 13-18 when their second most recent album came out. They moshed to Behemoth because of their stage act (which, in my opinion, was EXTREMELY gimmic-ish...they all look like Alice Cooper!). But between songs now and again they'd shout ridiculously overdone and underfelt anti-Christian phrases, and the younger guys love that stuff!
Anyways, sorry for the long post...I hope this helps you guys realize what "comes over" people to make them want to mosh.
Later!