time drag

For me, it's as simple as because you don't think about time when you are enjoying the moment and you do when you are bored.

When one is not looking at a clock and, hence, making a measurement, our personal metabolic time governs the rate at which time flows and this can only be in the forward direction. For example, when one is bored, visual V5 metabolic rate may tend to increase. Thus, one looks at one's watch more often and time appears to run slower. When one is relaxed and having a "good time", visual metabolic rate may tend to slow and time passes at a faster rate. So, one looks at one's watch less-often in this case. This is a quite common experience. Hence, the common expression "time flies when you're having fun".