Your lot in life -- a thought experiment
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The creator tests anything and EVERYTHING. Without doing this, we would not even have been here as evolution could not occur. If you want to find out as to which car is the best out of 100, you must give them all a test drive even if some of the cars have broken engines - there is the off chance that the fault in the engine could give it superpowers or something. For example, Daniel Tammet.
The reason you are you and not anybody else is very simple. If you were anyone else, you would be them and not you. We are a product of the DNA sequences in our parents' sex cells, this + environment defines us.
Of course this does not leave anyone fully satisfied because this is a very existential question. It's like asking why grass is green. Eventually all will be revealed.