im 19 so i just thought i'd add my thoughts on this ! when i was living at home, my room was pretty messy (clothes on the floor, magazines piled on my desk, make-up stains on the carpet etc) but not too unhygenic. i think it's just the sort of person i am that, if it's just me going to be in the room, im not too bothered with it being a bit messy because i've got lots of other things on my mind...
but it drove both my parents absolutely mad and we would have blazing rows about it i.e 'tidy your room! i can't stand to even go in there it's so messy!' me: 'well don't come in then! you don't need to !'(it was true that there was no real need for them to come in because i brought my own washing down and tea cups etc.)
i think it just annoyed them that they didn;t have complete control.... also the rest of the house was very cluttered with their stuff...i guess they felt since they couldn't keep ontop of their own clutter they wanted to have some sort of control over mine instead. maybe that's it.
anyway my point is that i was already stressed with g.c.s.e's, coursework, falling out with friends, boy problems and all those teenage things...so having my parents shout at me every day about something which i didn't place very much importance on (spickandspan-ness) and which in the rest of the house they didn't either...made me super stressed, and feel negative feelings towards them.
also, i guess when my room was at it's worst... it started to depress me and annoy me too and i wanted to tidy it but because it seemed like such a massive job i didn't know where to start! so at times like that my mum offered to tidy it with me and that was a big help.
also...the interesting thing is that as my younger brother was starting to go through that phase of having a messy bedroom (let me just say, worse than mine ever was! condom wrappers, fag ash, burnt chest of drawers, mouldy apple cores, toxic smell etc) they just left him to it and hardly ever nagged him about it !