To be a songwriter you need to have the mindset of one. Always be on the lookout for an idea.
I came across an article called "Write a Song Today - The Songwriter's Way of Thinking" by Shamir Rele which sums up how I feel about writing songs.
No matter whether your life is going good or bad, is eventful or not, you can always find a song in whatever you do... If you let it that is.
That's the songwriting mind Shamir is talking about.