I wouldn't over analyze alignment its just a rule to prevent extreme actions that don't make sense or give other players an idea of what a character or creature is supposed to be like. For instance, saying a goblin is lawful evil would tell you more about the personality in a nutshell rather than having to spell it out each time.
That's how I view it now Krusten, just making sure you are acting the part and not trying to be as though you're character is bi-polar.
Alignment is just a guide for character behavior but its not a brick wall so loosen up a bit and role play without wondering about if you in or out of your alignment.
While alignment is not something set in stone it is a guide on what your character believes in life. Sometimes things can and will come up that causes a character to act differently than their beliefs in life. As long as one atones for their actions all will good in the world.
Why does alignment need to [be pre-selected] or fixed.The choices of the character would determine the alignment which can actually change mid-game based on those choices.I hold the same view about classes.