Why we came ??

Today I was speaking to a friend and it reminded me of a question which I had since long but forgot about and stopped searching...

My friend told me that the purpose of this life is To get Moksha, To unite with God, To go back to that ONE because we are part of HIM and got separated...

And I do believe what he said is right as it is same as what all our great Gurus have also said. So it can't be wrong it has to be right... The purpose of this life must be going back..

But the question I have is that if we were initially part of HIM than why and how did we get separated and what is the proof that once we get moksha we will not get separated again. From what I have heard or read so far is that we were initially part of HIM but then suddenly something happened that we got separated (a. What exactly happened we don't know). Some people say it was His wish and He wanted to see many rupas of His own and thus created this universe. (b. But what is guarantee that once we get Moksha, He will not have such a wild wish again and separate us from Himself again). Others says that He was getting bored so He created universe (But again the same question what is guarantee that He will not feel bore again and separate us again)..

So frankly seeing I haven't heard of any right answer so far of why we got separated from HIM that now we our aim is to go back ? And I feel finding this answer is very important because until we get to root of problem we might not fix it and we may come back again. Secondly there is one more reason why I think finding answer to this question is important because may be He has send us here for some purpose (c. What is that purpose I am not sure ) and instead of completing that task for which He has send us, we are trying to go back to HIM and I don't think this should be right. We should instead put our whole heart and energy and completing that purpose and than He will automatically take us back.....