We all need motivation to write songs or to do anything. When I first started writing songs my motivation was to fill time at my gigs. I started playing at the coffee shop and I needed to fill three hours. This was alot of time for a rythmn guitar player who didn't claim to be a singer. I found out that it was easier to sing songs that I wrote than songs that other folks wrote. Basically, I wrote songs that did not got to high or too low up the musical scale. Every time I had a gig I would have a new song to play. Eventually I had more of my own material than cover songs. I was motivated to write my own material for my gigs. It worked for me and it might work for you.