This is one question that I've been trying to answer myself as I've personally been out of touch for so long. Mainly, my reason for not writing was due to laziness, but not only that, it was also because I was not inspired enough or did not have any ideas on what to write. Also, when I did have some ideas, the problem started when I wasn't able to write down my thoughts properly. As a result, I have not been writing for long.
Seeing this question made me think and for me personally, I was motivated now, after my long absence, because of the new changes to this site. hehe It's become so much better.
Not only that, I've started reading recently, and I've been getting some ideas on what I would really like to write about though of course, I'd have to do some research on them.
At the same time, the comments are another factor that really motivate me to write. It makes me happy that there are people who's actually enjoyed what I wrote or that I'm able to help people.
Of course, one other motivation is making money. It is slow for me, especially since I don't have many hubs and not all are what I would call useful or good hubs. But slowly but surely, my hubs are helping me to earn something and I even received payment once. My joy then was indescribable.
Also, what motivates me most is that there is no deadline for me to finish and I can write about whatever I like.
I hope that you are able to be motivated to write too.