I think the joy that comes from being a lawyer really depends on how you view the career that comes with it and the day-to-day work that you'll be doing.
For me personally it's really about the excitement of taking complex issues and boiling them down to simpler points that can be understood by a wide range of audiences. Of course, there are times when being a lawyer means staying up long nights and managing lots of expectations at once, but that's part of most professional careers and Law is no exception.
Your question is an excellent one. As Henry said, practicing law can be challenging and even exciting at times. But it can also be tedious, and demanding, and stressful. For the right person, that mix can be the source of satisfaction and a feeling of accomplishment. But, as demonstrated by the much higher incidence of substance abuse, addiction, anxiety and depression among lawyers, it can also be no fun at all. Many people go to law school and enter the practice of law because they want to earn a lot of money, but they find they do not really enjoy the work. Even for those who manage to make a lot of money, they often find themselves feeling unfulfilled and empty. The adversary nature of many areas of the law can also be wearing on one's soul. So I agree with Henry that one's attitudes, motivations, and personality, along with the nature and area of one's legal practice, will all determine whether one enjoys, or merely endures, the practice of law.