Once again, from Bob Dylan's 2004 Chronicles, Vol 1:
I can't say when it occurred to me to write my own songs. I couldn't have come up with anything comparable or halfway close to the folk song lyrics I was singing to define the way I felt about the world. I guess it happens to you by degrees. You don't just wake up one day and decide that you need to write songs, especially if you're a singer who has plenty of them and you're learning more every day.
Opportunities may come along for you to convert something---something that exists into something that didn't yet. That might be the beginning of it. Sometimes you just want to do things your way, want to see for yourself what lies behind the misty curtain. It's not like you see songs approaching and invite them in. It's not that easy.
You want to write songs that are bigger than life. You want to say something about strange things that have happened to you, strange things you have seen. You have to know and understand something and then go past the vernacular. The chilling precision that these old-timers used in coming up with their songs was no small thing. Sometimes you could hear a song and your mind jumps ahead. You see similar patterns in the ways that you were thinking about things. I never looked at songs as either "good" or "bad," only different kinds of good ones.