Who would have thought it? Certainly not me, but here’s my thoughts on why after doing it constantly for 5 days:
Firstly, and most importantly, you’re heading away from the crowds. As a general rule, the further up you go, the less people there are and you’re heading into the unknown. Downstream the population is bigger.
Secondly, going uphill means you see more. It’s more tiring, so every village is taken slowly and is often an excuse to stop and rest, maybe in a biergarten. Downhill is faster and you’re less inclined to stop, the scenery whizzes by.
Finally, you get fit quick!
a small fe 
Am not entirely convinced, if you had said you get to eat more chocolate…well, maybe then