After some reading and thought, an idea not at all unlike the one I read made itself clear to me. When fun is present, you focus more on the activity or person you happen to be enjoying said time with. As a result, the timeframe seems to be miniscule, and never enough.

However, when mental and/or physical discomfort is present, you focus more on the discomfort and anxiously await any release from this discomfort. In doing this, your thought process speeds up, causing more information to be processed than normal, making time itself seem to slow way down. This also works in the same way that people under the influence of marijuana perceive time as very slow. While general consensus says it slows the mind down, it has been my experience that it speeds it up. As a result, the faster the thought, the more information processed in a given time, and time seems to slow down.

This could also be related in the sense that for the old, hand-cranked cameras, the slower you crank it, the fewer frames per second are recorded, making the picture faster. As the operator speeds his cranking, more frames per second are recorded, giving a more detailed, slower version of what just happened.