Parallel universes, alternate dimensions, whatever you want to call them: the possibility of other worlds has infiltrated science and science fiction, to the point where most people have an opinion on the subject.
To read about the scientific debate behind the multiverse theory, read this Wikipedia article. I'm no scientist; I'd rather talk about the philosophical implications, if that's alright with you.
If this world is merely one of many, all slightly (or radically) different; if every choice sends us shooting down one leg of the trousers of time; what does this mean?
It means that whatever you do (killing someone, falling off a cliff, adopting a child) had to be done in some universe.
It means that, somewhere out there, Romeo and Juliet lived to a ripe old age, or never met.
It means that we can work around all the little ideological glitches of time travel.
It means that however remote the chances of life are, it was bound to happen somewhere, and that there are probably loads of unoccupied universes out there, and maybe many where dinosaurs still roam the earth, or where only Venus supports life.
So choice doesn't matter in the multiverse, because viewed from the outside it's just everything happening. Absolutely everything.
If, on the other hand, there is only one world, one universe, and this is it, well what does that mean?
It means what you do matters.
Your choices have consequences.
This is your one chance to get things right.
It means that something as colossally unlikely as life must have a purpose of some sort, and should therefore be valued and treated with respect.
Okay, so the multiverse theory is exciting, in a Star Trek kind of way. Other species, other stories, a world where you married Claudia Schiffer, anything and everything your imagination can concieve. But I personally find it much more thrilling to imagine that my life, my decisions, somehow count. I think it's better that way; better if we live knowing that the way we do things will be the way things are done, and that nothing can change that.
It just feels nicer, doesn't it? To know that you're not a murderer, you never were and hopefully never will be? Anywhere? Ever? To know that you can create yourself in your image, and that it matters?
I think it's a more beautiful idea, and that it's true.