How does the mind work?

It is amazing how someone my age (31) can feel so stinking old sometimes.  Granted I have plenty of life experiences.  I've seen most of this country.  I've seen most of Europe.  As have most people, I've experienced the deaths of many family members and close friends.   I have, and have had, more than my fair share of health problems.  I am married to the most amazing woman.  We have been married three years and together well over 4.  We have no kids, though.  Maybe my feeling so old is why I do not want any kids.  The fear of feeling even older.  But even as I inch closer to middle age, and I keep waiting for my biological clock to start ticking, I just do not want any.  I don't think I ever will.

I have been working with teenagers since I was 20 years old.  I began by volunteering as a big brother to a couple of kids as I finished my undergrad work.  As soon as I finished my Bachelors I went straight to in a residential treatment center very similar to where I work now.  Within two months of working there I had been bitten, scratched, and had a broom handle broken over my head.  Within the first year I had been punched in the face three times and been knocked unconcious for 6 hours by fire extinguisher wielding 17 year old.  During my time there I began to get to know "these kids" in a very general since.  All of them had a story.  Some bad.  Some really bad. 

Over the course of the next 8 years I moved on from this job to work with a couple of foster care agencies and a community mental health center.  I can not tell you how many hundreds of kids I have worked with.  Over the course of this time there has been that one question that I can not seem to find the answer to.  How does the mind work?  I know it is a very general question.  And there probably is no real answer.  I know there is a good understanding out there through the research of people way smarter than me that explains the biology and physiology of it.  But, that still doesn't answer my question. 

A Better Explanation of the Question
Every time I think of this question, two young ladies pop into my head.  Both were 16 when I first started working with them.  I worked with both of them for about a year.  Both had very very similar stories.  I will tell their stories as one.  IMHO, the differing details are irrelevant when posing the question mentioned above.  Their details- both have an older sister that was low functioning.  Both have a younger sister and an even younger brother.  Mother was a drug addict from the day they were born.  Father was in and out of jail.  When he was home, he was physically abusive.  Beginning at age 8 mom began prostituting them out for drug money.  Literally selling them to the local drug dealers as "sex slaves" in exchange for drugs.  Mom also prostituted herself out for drugs.  Many times the drug dealers would give mom more drugs if she forced the kids to watch mom literally get gangbanged and raped.  When these girls were 12, mom committed suicide.  Both girls came home from school to find mom.  With dad in jail again, and all family members wanting nothing to do with these kids, they ended up in the foster care system.  5 years later, and multiple foster homes later, I began working with these two girls.  Over the course of those 5 years, one of the girls ended up in psychiatric hospitalization 24 times.  She attempted suicide at least 13 times.  She also made innumerable threats to kill any and everyone she ever came in contact with.  She failed grades repeatedly.  She had no friends and was generally just plain scary.  The other young lady had some troubles as well.  But nothing that bad.  She was never hospitalized.  Never attempted or threatened to harm herself or anyone else.  When I started working with her she made good grades and was never in any trouble.  She was very well adjusted and a real pleasure to be around and work with.  Two very similar young ladies with near identical stories that ended up so very different.

So my question is this, how can these two girls with almost identical backgrounds turn out so different?  How does the mind do that?  How does the mind work?  Two Bachelors degrees, a Masters, and half way through a Doctorate and I can't figure it out.  I really don't think there was enough of a difference between the environmental factors of these two young ladies to explain it.  Obviously everyone is a little different biologically.  Is there that much of a difference though?  I know brain chemistry makes a difference as well.  But how can two minds assimilate and process through all of that information so very differently.  I know it happened, because I witnessed it.  But how? 

After years of witnessing first hand and hearing first hand accounts of things you would think only happen in movies like "The Hills Have Eyes", "Hostel", and "Saw", I still am blown away by the ability of these kids to cope with that.  I have to admit I have already gotten a little jaded to the stories.  Nothing surprises me any more.  I really can't remember the last time I heard a story and thought, "How do people do that to their own kids?"  Don't get me wrong, I still have eternal sympathy and empathy for these kids.  So many times I just want to give them a hug, promise them everything will be OK, and then some how force the entire world to make it OK.  But was does really continue to amaze me is that some of "these kids" can just roll with the punches, and some absolutely just crumble under all that pressure.  So tell me, how does the mind work?

Epilogue (of sorts)
I still wonder about those two girls.  Last I saw them one was going back the hospital, the other was just elected homecoming queen.  I'm a little scared to google either of them.  I don't think I really have a desire to know the truth.  I just like to daydream that the one got her life on track and that the other is somewhere in college on her way to medical school.