My Book? It's About People Who...

Why is it so hard to tell people what your book is about fast?  They ask, "What's it about?"  My memoir is about, it's about my, well, going into science.  Lame and lamer.  It's about having it all, but wait, some of it is about having doors slammed in your face and..  Lame too.  It's about trying to go into science before men wanted women to join them in the field.  Sounds formal, cold, doesn't imply beheading frozen flies to make a buck.  I don't think one sentence can do the job.  My novel, The Bad Project, is the same.  It's a coming of age story.  But wait, it's about college women.  No, it's about ethnicity. Not just ethnicity, about how friendship can reach across those barriers.  But is that the bottom layer of meaning?  Isn't it really about how, no matter what your motive is, looking for trouble is very successful?  Or is it about love and support between friends, how friendship is the most important take home message?  Here's the problem:  if you only had one sentence to say, why would you write a book?  Isn't it because you had more, deeper, more complicated, more interconneted, less obvious, more fun, funnier, sadder, than one sentence to say?  How can you strain out all the good parts you were trying to include and summarize the book in one sentence?  Something is bound to be lost.  Any ideas out there about how to do it successfully??