The scientist does not study nature because it is useful: he studies it because he delights in it, and he delights in it because it is beautiful. If nature were not beautiful, it would not be worth knowing, and if nature were not worth knowing, life would not be worth living.–Henri Poincare
As with any human endeavor, there are as many reasons for pursuing physics as there are physicists. This page will help you learn about some of these motivations and begin to develop your own reasons for a career in physics.
You might begin by meeting the people who do physics, or by exploring descriptions of physics, museums, and histories of science on the web. Or you may prefer to learn about the main branches of physics research. As you become more informed you will be interested in the professional organizations that physicists form.
If you are really serious about physics, you should start doing undergraduate research and see if you like it.