Your job focuses on your weaknesses.
Perhaps you started your current job under the impression that it would let you use your unique strengths to do fulfilling work. When you began work and started learning the ins and outs of your job, you realized that instead of playing to your strengths, the position requires skills, strengths or a disposition that aren't in line with who you are.
If you're in a position that plays more to your weaknesses than your strengths, is there a way to shift that balance? Perhaps you can learn new skills that make you better suited for -- and more satisfied in -- the job. Maybe there's another position within the company that suits your interests, and you can orchestrate a transition. If the difference between what you need to do and what you want to do is severe, however, you may benefit from talking with your supervisor.
Be honest with your boss: Explain that the job doesn't capitalize on your strengths, and you feel it's best to find an opportunity that better uses what you do well. If you've presented your case well, your boss may be willing to work with you and adapt your position, or help you move into a position within the company that better suits your strengths. This isn't always possible, but giving your boss the chance to help you before you leave will ensure that you're respected as a professional if you ever need to come back to your boss for a reference.