I can’t answer that question for other Lawyers but I know my answer changes over time.
I think we can get so caught up in working, that often times, it is later that we stop, breathe and reflect.  Take my Dad for example. He worked so hard for many years to support his family and prepare for the future. He retired at age 55.  I’s been mostly after he retired that we have talked and he shared stories about his mentors, career changes, etc.  I think this is because the years following his retirement he had the time to relax, think, reflect back, discuss and analyze.
So why did I become a Lawyer after being a therapist?  I think some people have formed prejudicial opinions about Lawyers.  Some have stereotyped those opinions to all Lawyers.  I never cared for negative Lawyer jokes.  They are usually from lawyers and I tell them I don’t appreciate the reflection on our profession.  In short, I am very proud to be a Lawyer.  I have seen the fruits of my labor.
My answer to why I became a Lawyer is:
I want to help people and it is extremely rewarding to have someone need a service and I can help.  Just today I had a former client from years ago, call me long distance in need of help.  Her relative was incarcerated and she was trying to assist but didn’t know where to turn. She needed a lawyer on a Sunday Morning immediately.  She also needed emotional support and someone to pick up the phone and say, “Hello, good to hear from you, how can I help?” I was there for her and that is my gain.  I was able to help, period.
I have great pride in my profession and I can count on many of my Lawyer friends, members of the Sarasota County Bar, members of the Tamapa Bay Bankruptcy Association and opposing-attorneys I have had worked with in the past, to be there for me on a Sunday Morning. They respond for the same reason I would respond to them during off hours – to help them help others.
The power and influence to make things happen for people in need of help is why I became a Lawyer.  Especially people taken advantage of because of an unjust change in the laws/Codes/Statutes, their age, finances, intelligence, psychological, or emotional condition. My vein of justice was ingrained from my upbringing and has been passed down to my children.
The more I grow, the more proud I am to be a Lawyer.