I feel uniquely qualified to speak to this issue because I did just that when I was in my mid-20's. I will not try to justify it, but will try to explain it. I have written about it before in my other blog here. The pertinent excerpt from that post follows:
At the same time, I followed [W] to his next assignment and ended all contact with my family and friends, including [J] and [Kurt]. What was I thinking? How could I do this? I believe I was running scared. I knew that I had pushed the boundaries beyond what anyone I knew and cared about would see as acceptable. I had done unthinkable things: taken up with a married man, a much older married man with children older than I, broken up his marriage, quit my job to be with him. Nobody could possibly understand those choices. I’m not sure I understood those choices. So I chose to disappear with no forwarding address.
I believe that there are times when people grow up in "good families" that are so "good" that they forget how to be loving and accepting or at least they forget to let their kids in on the fact that they can be loving and accepting. The pressure is on to be "good" to "behave" to "stay in line" and "not cause trouble." The parents may publicly praise their children for being such good kids, and tell the kids how proud they are. The kid (even as a grownup) hears this, internalizes it, thinks that if only the parents knew how bad the kid really was they couldn't possibly be loved. That's where I was. I made lots of mistakes (still do). I couldn't manage perfection. Yet I felt that perfection was the standard by which I was measured. So I left.
Eventually, I came back, but it took many years, and I am still defensive with my parents. There are still topics that remain off limits. I simply will not discuss with them my many "failures" in life -- being overweight, passing the CPA exam but not becoming licensed, marrying a divorced man almost 3 decades older than I.
I don't know if this helps anyone understand why some people run (and by no means do I think that is the only reason). I do know that I tell N often that no matter what kind of screw ups may happen I will always love him and care about him. I try to instill in him high standards but not expectations that he has to be perfect all the time.
And in case you hadn't noticed, this particular topic hit a real hot button for me.