This is me too. I have a couple of theories on this (surprise surprise - seems I do far too much thinking for my own good!)

I was put off smoking and alcohol having been forced to visit dying relatives in hospital who were there through smoking or drinking. Not a pleasant experience, but I guess it had the effect my parents were looking for.

On top of that, I can't stand the possibility of losing control if I am drunk. This has, in fact never happened - I'm unfortunately one of those people who can get totally plastered, yet still remember exactly what I and everyone around me did. This kind of defeats the object of getting drunk to forget!!!

I also have a fear of being physically sick, having been reprimanded for being ill when I was younger - ruins the sheets apparently (how unfair is that on a kid?).

I also have a need to be able to run like hell if the need arises.

Anyway, as you can probably guess, I am usually the designated driver whenever I actually take the risk to go out anywhere. My friends accept that I will give them a lift in, but if it's a late night in town, everyone gets there own way home. This means that I can leave if/when I need to without feeling bad about abandoning my friends.

I'm really just a control freak!