Really good topic.
There are a lot of guiding principles for me. A lot of songwriting is instinctive and I have recently had to verbalize it for my sixteen year old son who is now doing very well at guitar and singing.
The melody should move you emotionally.
You should be able to transfer that emotion to the listener. They should feel the same emotion you felt when writing the song.
The music (chords and melody) should fit the words. #1 rule in my book.
There should be drama,..a building to the first chorus and the second and the bridge (if the song is layed out that way) to the ending.
The drama should be created both by words and musically. (Adding embelishments as you go on, or volume dynamics, maybe a suprize ending or moral, etc.)
Music is emotion.
Songwriting can tell a story or capture a moment. And for a moment, it can change those who write it and those who hear it.
I usually start out with a piece of a chord progression or a small thing I've learned to play on guitar and play it over and over and ask myself, What does this sound like? What does it feel like?
It may be a song about a person who doesn't exist. That doesn't matter to me.
What that person would feel if they did exist is what matters.
Mike said for him it is like a wrestling match. For me it is like a puzzle. All the pieces have to fit.
Bob