If you've put pants on and worn them they become dirty. If you then have a shower why would you put the same pair on again? 
I have carried out a kennel wide investigation and whilst the THW is happy to have dirty knicks pilling up under her bed for evermore Mrs S agrees with you word for word Wenchy and that was before I told her what you had said, although she did think that 200 pairs was probably a tad too many ("bordering on the obsessional" I think was her phrase)
Reason for wearing jeans, at my advanced age, is that I belong to a generation that believes that Jeans = No underwear or socks ..... trouble is I am now at an age when I cannot remember who it was who first advocated that theory so you'll just have to take my word for it.
