From what I see around me (my friends included) I'm kind of blaming it on alcahol... it corrupts!
For example, I have a friend of mine, when she started the job she was sweet, innocent, had a steady boyfriend... fast forward a few months and a load of nights out on the razzle and *BAM, one night stand.
One turns into two, two turns into three... etc etc.
Suddenly she's loving the 'fun life' Years later she matures and realises what a big mistake she made, by that time its to late to get what she had back.