Note: If you are squeamish, or think that talking about bowel movements shows incredibly poor taste, then read no further. Don't say I didn't warn you.
There's nothing quite like it. That weird sense of release, the strange squirming feeling as it escapes. It's new, different, odd, because you're so used to having solid bowel movements, so it can be a little unnerving. But if you have managed to make it to a toilet, and don't need to go anywhere for a while, it's a fascinating experience. I should point out though, that if you are not in the immediate vicinity of a toilet, diarrhoea is the worst thing in the world. When you feel that funny tingle in your stomach - the one that quietly whispers "Get to a toilet RIGHT NOW, man, I'm not even joking with you, or you will shit your pants and leave muddy footprints" - you probably have less than five minutes to get on that throne. And God have mercy on your soul if you don't.
You sit down. At first, you don't want to let go - it feels weird, like your intestines are about to explode out of your arse. When you eventually let it flow, there is a burst, an ugly noise, and you are shocked to feel that it is liquid, like you are pissing out of the wrong hole. It's not an unpleasant feeling, almost a relief that it is so easy - all you have to do is just relax, and it takes care of itself. It's quite entertaining, and makes some wonderful sounds.
It is around this time that one of the only two bad things about diarrhoea occurs to you (I'll get to the other one later): the smell. Oh sweet Jesus, the smell. Obviously, if you have got the runs, the squits, the thrupenny bits, the Jimmy Riddles, the scutters, the bowl pebbledasher, then there is something wrong with your insides. Food poisoning, dysentery, some sort of gastric problem, whatever - diarrhoea is not the normal run of things. Illnesses usually produce some sort of bad smell somewhere, but the shits take the gold medal for that. It's a sharp, acrid abomination, and it hits you like a slap in the face. Normally you can stand your own shit stink, appreciate it, even, but not this one. This one is rank. Eventually though, this passes. It's still there, but you've got used to it. Try coming back into the bathroom later on, after it's all over, and you can enjoy it all over again. Other people will be horrified by it, and will look at you in shock all day, staggered that someone they know, someone they like, could have produced such a foul stench.
After a while, you enter a zen-like state, calmly thinking about your life. Every now and then, there is another little explosion. It's not a constant stream, as you'd expect, so if you have the time to spare, you may as well stay where you are and get it out. Why do I enjoy it so much, am I some sort of pervert? No. I enjoy it because it is so much better than being constipated. When I'm constipated, I just want to fucking die. Diarrhoea, on the other hand, is easy, relaxing, and fun in a bizarre kind of way. Sue me.
When you think it is all over, you stand up to survey the damage. There are two kinds of diarrhoea: little blobs in a watery soup, and the pure, liquidy type. The first one flushes away quite easily, maybe you get the odd floater, but a second flush takes care of this. The second type is like a chocolate pudding, it floats on the top of the water, and takes approximately 9053 flushes before it even starts to consider shifting.
It is now that we come to the second bad thing about diarrhoea, arguably the worse of the two: the cleanup operation. Normally, arse-wiping is a fairly simple operation. Not this time. Your bumcheeks are caked with the filthy stuff, and you can easily go through half a roll of toilet paper before you start to feel remotely clean. If you happen to be of a hirsute nature, it's even worse: Liquid shit and arse hair do not mix well. Or rather, they do mix well, which is the problem. But let's not dwell on that.
Finally, it is all over. As illnesses go, it is by far the easiest to deal with. As long as you can stay at home, get to the bathroom quickly, and keep everyone out of the house, then you have a fairly easy time of it. How many illnesses simply involve sitting down for a while? You can even catch up on your reading. Of course, if it is merely a symptom of a bigger disease, like dysentery, then my simplistic analysis doesn't hold. If it lingers, or you feel any other ill effects, please see a doctor. But if you have merely got the shits one day for whatever reason, then that's okay. Sure, there are more pleasant ways of spending a day, but this one is free.
One word of warning, if this is your first time experiencing the runny-bottom madness: Be careful. Be absolutely sure that you have finished. If, later on in the day, you feel like farting - don't. Go and sit on the toilet, and then fart. Nine times out of ten it will just be a fart. But it is that tenth time out of ten that you will thank me for.