Japan has a lot of earthquakes because there's this giant catfish curled up under the islands and when it moves, earthquakes happen... NO, of course not, but that's an old Japanese myth.
The real reason why Japan has so many earthquakes is because of where it is on the globe. Japan sits on an area where two tectonic plates meet. Tectonic plates are, of course, the plates on which the earth's continents and oceans sit. The plates float on the molten rock below. When they move, earthquakes can happen.
Japan is on the place where the North American plate and the Eurasian plate meet. It is also near to the Philippine and Pacific plates. The first two are more important because they meet right under Honshu -- the main island of Japan.
Why earthquakes occur and where they occur is explained by scientists by a theory called plate tectonics. The earth's outer shell is divided in about 10 large and 20 smaller. Each such plate called tectonic plate one consists of a section of earth's crust plus a portion of mantle. These plates slide or move slowly and continuously over a layer of hot and soft rocks in the mantle called asthenosphere. In this process the plates collide, move apart or slide past each other. Due to this relative movement tectonic plates stresses are created in the rocks along their edges. This produces zones of faults around the boundaries of tectonic plates. When these stresses builds up in the rocks in the fault zones the sudden movement of rock take place. This causes earthquakes.
From the above discussion it is clear that earthquakes are most likely occur in places that lie over this fault zone. Japan happen to be situated along one such major fault zone and this is the reason why earthquakes are more common in Japan