I'm coming at this from an American Old-Time music view point, so I can only speak to my personal motivations/thoughts and they're pretty much colored by that perspective.
Old-time is not really a performance oriented music (though I and many of my fellows do perform), but more of a community oriented thing. So for me, it's all about sharing. Sharing the musical conversation with those with whom I'm playing in a session; sharing the energy with the dancers I'm playing for; and, to a lesser extent, sharing the tunes with whatever listening audience there may be.
Of course, fun is a tremendously motivating factor, too. The physicality of muscles interacting with the fiddle, banjo, uke, or dulcimer is something I get great joy from and don't know that I could live without. As is the physical feeling of being surrounded by the vibrations of the music. I mentioned conversation earlier and that's part of the fun, too. When I'm playing with a few other folks and we're really cooking, the level of communication completely apart from the words we're speaking can be more fun and uplifting than any other form of communication I have yet experienced. When it's a more intimate and intense conversation between me and one other, knee to knee, eye to eye, spinning a tune together... It beats anything words could say.
And when the communication between the band and a couple of dozen dancers is going well -- the tempos are right, the dancers are hitting the same groove that we are -- it's at least as good as, and maybe better than good food, drink or even sex. Or at least it's good in a different way.
So, there you go. That's why I play tunes.