So I have this theory. It's not much, doesn't really stand up to any kind of arguments, but it's fun, simple and kinda makes sense. And heck, you can always use it as an excuse to stop fantasizing and just get on with it already.
Let's start with the basic multiple-universes-by-decision tree. I just made up that name, I have no idea what it's called. The idea -- and I make it very basic here because I'm no expert on the subject and we have better things to do than research this stuff -- I like to explain this way:
Think of your life as a line. From birth to death, a straight line. On that line, your life happens. Here you have a birthday, there you buy a house, somewhere else you live happily ever after. Now consider this: every moment of our lives we are confronted with choices and we make decisions. The decisions we make are what creates this line. But in parallel universes, you pick different options. So each decision point on your line is a point where a number of different "life lines" emerge, giving it the look of branches on a tree. On each line come multiple decision points that each branch off with their own option. Soon, you have a pretty large tree; one "branch" of that tree is your life, the other branches are parallel universes. In most universes life is the same, you just have a silver car instead of a black one. In some universes, wars rage because something you did (or didn't do) had that kind of "butterfly effect". You get the picture.
So the idea, to summarize it, is that every decision you make spawns a parallel universe in which things happens just a little differently. And here's where I make the big jump: the chain of events is such that only specific events can happen from a decision, whereas all other events are constrained to other universes. Does that even make sense? Ok, here's a completely fictional example that I never thought of before: you drive down the street in your silver car right by a woman that completely fails to notice you. In a parallel universe, the one in which you got the black car, you drive down the same street and your car catches her eye, you meet, talk, fall in love, get married, have kids and live happily ever after. Having a silver car means there is no way you can meet her (at least, not at that point in time).
Oy, am I getting deep in there. I'm trying to explain it in a way that makes some sense. Hope it's working. And keep remembering: this is just for fun, so stop trying to prove that it's completely impossible...
Now for the fun part.
Consider, just consider, that all this stuff is true: making a decision influences what will happen (duh) and what has the possibility of happening. Now consider this: what if your imagination is an integral part of the universe and thinking about making a decision is just as valid as actually making it?
It would mean that every time you imagine a situation, and imagine the decision you would make, you create a point on your life line from which all other decisions depend. Since a decision point affects what has the possibility of happening, a bunch of possible outcomes have just been moved from your universe into some parallel ones, just outside of your reach.
In other words, when you sit there and think of that cute guy you like and picture yourself walking up to him, engaging in conversation and saying exactly the right thing (and, of course, I'm talking about someone else... I would never walk up to a cute guy, obviously) what ends up happening is that his answers and the conversation that follows get split off into a parallel universe, just because of your thinking about it, and makes the whole thing impossible to actually happen.
Which is why, after having thought of walking up to the cute guy and having a thousand different perfect conversations, you walk up to him and say the absolute stupidest thing. You have no choice! That is the only option left in your universe, all the good lines have been "moved" to parallel ones!
So there. That's my theory on why you should just stop day dreaming about stuff and go and do them already. Because the more you think about how it could happen, the less likely it is to happen that way.
That, or start day dreaming about the thousand different ways you could look completely stupid. Maybe all that will be left will be the perfect conversation, who knows. I personally never tried it, so let me know of your experiences...
