An interesting post, so many kindred spirits here.

For a great many years paddling defined me as a person and I was often asked why do you do it? Especially the long solo expeditions.

After giving it some thought, this is my long and slightly convoluted answer.  :)

I always found on the really long solo trips it took me at least a month to finally shed the trappings of civilization and truly become in tune with my environment. Did I ever feel lonely? No. traveling every day, experiencing new sights and vistas around every bend kept me constantly engaged and filled with wonder.
Time became irrelevant. I slept when I was tired, ate when I was hungry and I was beholden to no one.
The ability to carry everything I needed to function happily on my back gave me an incredible sense of freedom. Food, clothing, shelter, transportation - all distilled down to it's most basic form. More than anything those long trips taught me just how little we as humans need to be happy and content.
That lesson has stuck with me and has fundamentally changed me as a person. To this day I value experiences far more than things and always will.
The spiritual aspect of it has been mentioned and it's very true. How can one not feel it when faced with the magnificence of the landscape you're traveling through? To become intimately connected to that very same environment is as close to a religious experience as I'll ever have.
That aside, doing those long solo trips was empowering, instilling in me a deep seated sense of self confidence. Basically, "if I can do this, well then I can do anything!" Those trips changed my entire outlook on life and gave me the confidence to shape it in the way that I wanted.
Every man is the master of his own destiny. Mine has and always will revolve around wilderness.