the paradox of time

Why does time drag on when things are dull, or fly by when things are exciting?  A conversation with a friend that seems likes minutes and then a quick pause that prompts a glance at the clock shows that hours have gone by.  When time passes quickly, I associate this with positive experience.  When I actually notice time, it's generally when an experience is unpleasant or my mindstate is that of impatience.  Lately, I've expanded my mindfulness practice into doing a 20 minute meditation every morning and evening.  At first, it was difficult, my mind would race with thoughts and worries and I had to keep refocusing on my breath.  Sometimes (and it still happens now too, just not as often) I would feel so defeated because the time would pass so quickly while my mind was lost in a maze of useless thoughts.  But the sessions when I'm able to truly let my mind take a break, the time passes slowly, yet this time, instead of feeling like I'm waiting for water to boil, the slowness and stillness is precious and sweet.  And when it's over, I feel like it passed too quickly.  huh.