Answer:
I grow up while growing down
My first though water cures no thirst my next alone has soul and when he lives upon my first he then is called my whole.
I start new then become old. Start clean but be one dirty. I usually start big then become little.
Deep, deep, do they go. Spreading out as they go. Never needing any air. They are sometimes as fine as hair.
Sometimes I am white, sometimes I am dark. Most people love me. What am I?
Runs over fields and woods all day. Under the bed at night sits not alone, With long tongue hanging out, Resting at your feet until we go for a walk