Did you ever know something was true, but you didn't really get it until it finally hit you?
Writers write. Duh.
I have a friend who is a novelist. She writes pretty much every day, even when she has worked all day at the job that pays her much more than writing does. She writes whether she feels like it or not. She writes whether she feels inspired or not. She says, today I will finish X number of pages, and she does.
Writing songs isn't the same as writing novels, but it's not as different as I have treated it. I have written songs when the muse hit. I have written songs when they came tumbling out in one sitting. Sometimes I have written songs that took a long time to finish, but mostly because I worked on them sporadically.
That's not what songwriters do. The job of those who worked in the Brill Building (just one example) was to write songs. So that's what they did. In the process, I imagine they wrote and then tossed out a lot of crap that we never heard, but they also came up with some of the most memorable songs of all time.
My friend can write X number of words because she already had her novel plotted out and her character arcs set. She has a framework. Coming up with three-minute musical gems is different. There are a lot of aspects to the process. There's the subject, if you have one (some songwriters are really good at just writing words that sound good together but only seem to mean something). There are words, rhymes, phrasing. You want a killer chorus or a memorable refrain, usually, although it depends on the song. And then there's all the music—melody, more phrasing, rhythm, chords. Every song starts somewhere and grows from there.
Not every idea turns into a good song. Some you won't finish. Some you'll finish but throw out. But other than the rare song that writes itself, there's really only one way to write a good song—work at it.
That's what I have to do.
No doubt I will continue to scribble down lyrics that come to me whilst I'm stuck in traffic. No doubt I'll get melodies in my head when I'm not really able to grab my guitar and figure out where they go. But what I really need to do is block out time, just like my novelist friend does. And then maybe those traffic jam scribbles will get finished instead of just sitting there on a scrap of paper.