Because the law, especially criminal law, is a magnificent, ever changing, living thing.... an instrument that, if played well, can make our society a better, freer, safer one. The task of the defense lawyer, that of making sure the prosecution PROVES guilt, beyond reasonable doubt, and of ensuring that the trial is a fair one, that is among the noblest things to which a human can devote themselves.
Anyone who can't understand why you'd study law does not deserve an explanation.