It's probably a fair assumption that everyone who has stumbled upon this blog either has an answer or is searching for one. I for one can attest that this question has been the most important one I've faced in my academic career; perhaps it will be for you as well.
There's some easy answers. Like job opportunities and security, fulfilling graduation requirements, or pure and simple bragging rights.
But the simple answers aren't necessarily the best answers...
You see, these are extrinsically motivated goals. These motivations come from outside sources. They're "carrots" given to you by future employers, teachers, your peers, etc. and aren't coming from yourself. The most gratifying fulfillment will always come from within.
Intrinsic motivations are the kind that come from your soul. Some examples of intrinsically motivated goals are gaining insight into a culture that interests you, having a healthier brain as you age, desiring a closer bond with native Japanese friends or family, and much more.
Completing intrinsically motivated goals don't always grant you any financial, political, social, or academic benefits. They can lead you to such benefits (as fluency in a second language will certainly give you), but it's frankly just icing on your self-improvement cake.
I got the following advice from one of my professors. Take a moment now to write all the reasons you have to learn Japanese. Once you have them all on paper, make a note of the three to five reasons that are most important to you.
If those most important reasons happen to be intrinsically motivated, then that's perfect! You have a good foundation to begin learning!
If they were extrinsically motivated (or if you had a difficult time finding any reasons), then you may want to reconsider your plans. I don't say this to scare you or to give you the impression that learning a language is impossible without intrinsic motivation. I simply wish to give you fair warning that language learning is duller, more difficult, and ultimately less fulfilling.
Now, write your reasons down on sticky notes. All of them will help, but pay close attention to your important ones. Display them everywhere! Put them every place you have to look at each day. The corner of your mirror, on notebooks, above doorknobs, on the fridge, and by your key-hook are a few ideas to start.
The goal is to remind you constantly of why you are studying. This will help you stick to your goals, especially when things get rough.
Having your motivations proudly displayed will set the stage for your academic success!