Why is it so hard to ride the trainer?

Does anyone else have this problem? It always feels so hard to push the pace on the trainer. I did what felt like a HARD workout this morning but when I looked at the data I straddled zones 1 and 2 for most of duration. I hit a few minutes of zone 3 and it felt like sprint triathlon race pace... just plain brutal. Had I done the same workout outside while maintaining the same perceived exertion I probably would have averaged more of a mid-zone 3 workout.

Everyone seems to report the same feeling (though perhaps not quantified by zones, power outputs, etc). Why is there such a mismatch between the perceived exertion of riding the trainer vs riding outside? I have a few theories as to what contributes to this disparity but I want to hear your own too.

• Motivation and focus - When you ride outside it's probably either with a group or maybe on a path/road that has other riders. Riding with others seems to add significant motivation. You end up focusing on things like catching that rider or keeping pace with your friends. When you're alone riding the trainer you have almost nothing to focus on but yourself and your own pain.
• Highly repetitive motion - When you're locked in that trainer, there's no steering, there are no bumps, and there's no need to navigate around obstacles, react, or switch positions. You're just stuck in that one position doing that same motion over and over. You start feeling every single imbalance... a weird feeling in your hip, a tight quad muscle on one side but not the other, tight calves, etc. The repetitive motion and static position seems to amplify the effect of these conditions.

So tell me... what do you think? I want to hear your thoughts.