Two things motivate me:

I usually treat myself to a good breakfast before my ride...can't wait for that croissant and latte....and I feel I "earn" that breakfast if i ride 20 or 30 miles afterwards.

I remember what it was like to be a kid again while riding my bike down country roads. That sense of freedom and carefreeness is very motivating to me. Such a simple wonderful pleasure, pedaling along past the cornfields, past babbling streams, kids playing in their yards, cats sunning themselves on porches.... It's so good for both the body and the soul.