Interesting thread! (even though I can't read the Japanese parts... XD)
I agree with Elizabeth that it's a balance of different things that make the motivation for me.
There's what you might call the initial motivation, that kicked it off - y'know, the thing that originally made you decide to learn Japanese. In my case, it went something like this:
-- Interested to understand song lyrics.
-- Began to find myself 'learning' inadvertently, in the same way as I started learning English as a child - my brain started by itself to figure out sound meanings, grammer and parts of sentences.
-- Decided to formalise that by getting a book and starting classes.
-- Still going!!! ^^
The motivations that keep me going are:
-- Interest in the 'intellectual' side of learning a language - the 'brain exercise' if you like. Languages - including my native language - have always fascinated me.
-- Being able to see progress.
-- Being able to communicate with someone (I'm actually thinking of a specific person here) in their own language. It makes better social skills, and communication would of course be the main motivator for someone actually living in Japan!
But I think the main motivator for anyone learning, for whatever reason, is being able to see and benefit from your own progress - whether that's being able to hold a more advanced conversation (with your fellow students, tutor, friends or work colleagues), read more kanji, understand a more advanced piece of writing, pick out some words in your favourite manga, recognise a sentence in a song... whatever. If you feel like you're not getting anywhere, it seems like 'what's the point in trying?'
In my case, although I can't read any kanji yet, I get ridiculously childishly pleased when I can understand some katakana words on the back of an eyeshadow pack or on a poster, and when I can understand a word or whole sentence in a song or an interview!
But although my level is low, there's also the hope of things to come that is a big motivator for me - for example, I'm looking forward to learning kanji even though I don't know when I will begin this. To feel there is a lot to learn in the future makes a spur to me, rather than a discouragement. But maybe not everybody's mind works like that................?