[ul]I can’t sing sitting here at my computer. Surely that doesn’t count. :D[/ul]
This is good for aerobic exercise, but not for weightlifting. I can talk a little during my intense sets, but generally only enough to say “I’ll talk to you in a minute” or something like that.
I enjoy my exercise but I think this is a behavioral mod rather than anything that intrinsically stimulates the pleasure center like, say, sex. Our bodies are from a period in evolution when we had to exercise as a survival strategy. Evolution had no need to make it particularly pleasurable, because we had to do it to live. Evolution provided sex with a big pleasure hit because otherwise we probably wouldn’t bother. A prior poster alluded to endorphins, which I think not so much provide pleasure but just sort of deaden the pain.
Exercise is based on the idea that you exert your body in certain ways, and it becomes stronger by conditioning to the pain and the movement of those motions. When you are a cross-country runner, for instance, you (usually) have very little body fat and not much developed muscle; the running has conditioned your body to a degree where it’s almost been streamlined. The same thing stands for weight lifting–you lift weights with a certain muscle group; the muscles become weak and damaged; they re-structure themselves stronger than before to adapt to the pain that was given to them. Micro-evolution in action.
Now as for the question: very few things that are good for us are comfortable. Sitting around at a computer all day playing games, or watching television, or events like that are all very comfortable and easy–but not good for you. Things that hurt–physical training, working (in some jobs), things that <i>challenge</i> you–they all hurt, but are often better for you. Pain isn’t something to be afraid of; it’s something you overcome. Pain makes you a stronger person, and a part of exercise is exactly that–devoted to making you stronger in body.
Well, that was spoken like a true Marine. My brainwashing is still in effect from basic training. 
When I surf in 52deg ocean water here for 5 hours & get out I feel great!..then after about an hour,
cold then sleepy, then fine again.