My theory on why so many famous people find themselves in situations like this:
Famous stars can have virtually anything they want, just by making a phone call, or nodding their head. Men, women, people, are constant throwing themselves at them, just for the chance to say they've been with them (certainly, things like this don't normally happen to the rest of us). Eventually, with so many different "opportunities" being offered to them, they grow bored with what has always satisfied them before, and need a new "thrill" to up the ante. This could take the form of sex, drugs, alcohol, etc.
There are some grounded famous people, but they seem to be the exception, rather than the norm.