Writing music is about hard work - about craft; about determination. Yes, to some extent talent helps, but it's certainly not like the popular notion of selling your soul to the devil for music, or a druggie experience where you get high and music flows into you.
If anything, the devil destroys the music - since he comes to "kill and to destroy", and so of course, anyone who sells their soul to the devil for music will never find good music in the end. But i digress.

It's about hard work, really. There's a pattern of music, and theory - and if you learn hard enough, and try your hand at writing tunes, and keep refining them, each little part of it, you'd start to know what works, and your intuitive sense, as you get better at the music bit, will improve, and you'll be able to think of what should melodically come next to a phrase, you know. So it's relaly hard work. because the brain is like that - the more you write, the better you get, which means you write better and you enjoy more, and so it's a good exponential cycle. The reason why paul mccartney was so good because he really works hard at his craft. All the great songwriters really work. It's hard work. Richard Rodgers would write everyday - to keep up the craft. Same for ...well.. any songwriter out there. Rachael yamagata, you name it. NO one ever becomes a good songwriter by being lazy. Seriously.