My Personal Purpose/Mission in Life  
In my writing and consulting, and teaching, I encourage people to use the "charter" framework to organize their strategic thinking.  In my experience many executives are confused about strategy and tend to mix up terminology and perpetuate confusion in their own minds and in the minds of their people.  The charter framework helps address this problem.  There are chapters on Charters in my book, Level Three Leadership, and available from Darden Business Publishing.  
In general charters consist of six elements:  a mission/purpose statement, a vision statement, a values statement, a strategy, short-term operating goals (which tend to crowd out all the rest in modern management, and leadership--that is, WHO is going to make all of these decisions and WHO will clarify them?  
One can make charters for virtually any endeavor--for a nation, for an organization, for a program or project, for a family, and for an individual.  In my consulting work, we tend to focus on ORGANIZATIONAL, WORK GROUP, and INDIVIDUAL.  Some corporate people will gag at this suggestion--given their experience in the last twenty years with laminated mission statements that read, "to provide world-class goods and services that delight our customers beyond their expectations and provide our investors with an above-average return on their investments."  This is an example of a mission statement that is true--and lifeless, uninspiring, energy sucking, and demoralizing.  This is NOT what I'm proposing here.  
I won't display my entire charter here, and some find it helpful to have an example of a personal mission statement.  So, here's what it is for me:
My personal mission or purpose in life is "to help people find themselves."  This stems from the fact that I've had three last names during my lifetime and have often struggled with "who am I?"  I wasn't able to articulate this purpose at the outset, and it's become increasingly clear over the years as I left banking, went into teaching and consulting. 

I begin with myself, then extend this purpose to my wife who was abused as a child to the point that she was very tiny emotionally when we met.  By virtue of our relationship and her own courage, she has blossomed into a vibrant, vivacious, outgoing adult.  Although that will never appear on my resume, it's the thing that I'm most proud of, having helped her along the way and having tried to provide opportunities for that to happen.

Next, we have four children.  Can we help them find themselves?  Each has their own set of issues and challenges, some bio-chemical, some emotional, some physical.  How will they spend their lives?  What will be their central endeavors?  It looks like acting, event planning, biology (marine and human), and photography at the moment.  What a wide spread.

Then come my students in the MBA program.  I've taught a Career Management and Self Assessment and Career Development course for thirty years--all focused on helping them find out who they are and what they want to become.

Then the participants in my executive education programs.  They come to learn, and I've become convinced that the best thing they can learn is clarity about who they are and how their VABEs affect their behavior as individuals and managers and leaders.

Then my consulting clients--who like Executive Education participants--seek help in figuring out what to do.  Who are we?  What should we do?  How should we do it?

Finally, there are my readers, those who are kind enough like you to spend a minute or two ingesting the words and ideas I offer.  My goal there as well is to help readers extend and expand their thinking and understanding and in essence clarify who they are and what they can and want to do.

So everything I do including my recreation revolves around this central purpose, to help people find themselves.

I have a colleague who says his purpose in life is to cause people to think.  Brilliant. 

What's your purpose in life?  I encourage you to go beyond "help people" or "be happy" or "raise my family" or "glorify God."  You're probably spending 20-30-40 years in a career or profession.  If you've spent 20 years building airplanes and you don't include something about aerospace or aircraft in your mission statement, you'd probably be fooling yourself.  At a minimum, it would look disingenuous to onlookers. 

I'm blessed or privileged or lucky to have found or chosen a career that allows me or perhaps the other way around taught me what my life's purpose was/is.  Academia, teaching, consulting and writing all fit nicely a purpose of "helping others to find themselves." 

Can you find the words to clarify YOUR purpose in life?  It's not a trivial thing.  Lily Tomlin, the American comedienne, once said, "I always wanted to be somebody.  I guess I should have been more specific."  What a great line.  Can you clarify your purpose in life?  It's the first step in self leadership.  In fact, I think one of the main responsibilities of leadership is to clarify the elements of a charter, at any level, to those around.