I've played guitar since I was 10 (I'll be 48 in September). When I started, it was because I was planning for my future and figured it would be useful in getting laid (yes, I thought about that a great deal when I was 10).
In my late teen years, it became a form of expression for me when I started writing a few songs. Throughout, though, it has been a means of elevating my mood and transforming my state of mind.
Something happens when I play. I don't know what it is, but once I've had the worst day in the world, a couple of hours of band practice makes me feel like the king of the world.
The other aspect of my musicial experience is recording and making a "permanent record" of the things I play. Although I rarely record my own material, helping to bring my collaborator's creations to life through arranging, engineering, mixing, etc over the past decade or so has given me a new respect for the skill of those who do this professionally, and just how much work goes into the things we take for granted on the radio and in our music collections.
The final key of this is the one that no one talks about much...ego. There's no bigger rush than to have people applaud a performance, or tell you that something you wrote or recorded or played on affected them in some positive way. Ego strokes make me purr...
I hope to improve my drumming and keyboard skills in the coming years, because my hands are becoming so arthritic and painful that playing my guitars and bass is becoming very difficult, and my proficiency is suffering. Whatever happens, though, I will make some kind of music, somehow, some way.