Why Are Some Answers Approved While Other Answers Aren't?

You've asked a very interesting question about answering on this site. You want to know that why some answers are accepted and approved while others are rejected. What is the reason behind? In fact, you must know that purpose of this site is to enlighten the people. So in this regard, we've to answer totally logically and with true facts. Moreover there must not be any copy of some information. You may search for some information but you have to write it in your own words. In this case your questions will not be rejected I ensure you.
Now coming to the second point of your question that why some answers is accepted. It has the same reason. If you've shoot out an answer with correct knowledge and true facts then in this case, you must be sure that your answer will be approved. And this is the reason of approving some answer.
So you must be careful while you're answering some question. Please make it sure that you're not answering a question twice because in this case your answer will also be rejected. And it is due to duplication of the answer. I hope it is very much clear to you now. And now you can understand the reasons behind them.