up and down are two radically different realities.
your body feels the difference between take-off and landing in an airplane, as it does on the up and down in a roller coaster car. we don't feel it quite as much in an automobile, but we often experience the difference greatly while in trams and gondolas and funiculars.
in all of those things, however, you are relatively passive, and thus miss out on experiencing the prodigious difference between up and down on a bike.
it is most noticeable during the days of lower range temperatures, say 40 to 55 degrees (since I am now a spoiled cyclist who rarely ventures out when it's under 40): going up the hill is a completely different experience, temperature wise, than coming down that same hill.
I know this is obvious to any thinking human, but unless you're a pretty serious cyclist, (and one who is willing to go quite fast on the descents) you cannot understand just how significant the difference is.
climbing up emigration on a 50 degree day I can be sweating like crazy, beads rolling down my back and slipping down my forehead. I can wear a short sleeved jersey and shorts, fingerless gloves and nothing under my helmet. I will be toasty and grateful for the cool air.
but before I descend I add armwarmers and perhaps a wind jacket, possibly even full gloves, and maybe even my headband. and I might be cold on the way down.
I rode yesterday and carried a light wind jacket --- just in case --- and wore armwarmers with my short sleeved jersey. a quarter of the way up the canyon I peeled the armwarmers down to my wrists, and when I reached the summit I pulled them back up for the next descent. when I reached bottom and began the next climb, I rolled those things back down to my wrists.
those armwarmers went up and down 3 or 4 times throughout my ride, and I was grateful to have them.
I smile on my way down the canyon when I am bundled up and pass those heading uphill in short sleeves:
they are experiencing a completely different reality from mine.
which is what I need to keep in mind when I pass people everywhere else: chances are, they are all experiencing realities that are entirely different from mine.
I'll smile at them, too.