Why sometimes people are in love but not together.

The scientific reason for this is
Because everything is fixed in Universe.
We can't change our past or future.
May be one day it will be possible to trevell in future and past,but we will not be able to change it.
This answer is not on basis of any science fiction movie.This answer is basis on my study and experiences.
You can go anywhere in the world most of the people are in this situation.
Specially this query born in teenager when he or she was high school student.
All that thing remind him his weakness like lack of money,having not very good personality, not having charming body.
But they will forget all the matter and his love once they complete their education. And this is good for them because if they go on thinking about their past they will be get only disappointment frome it.
They should not think about the past because everyone has some mistakes in his past.And if they remamber thier past they will automatically forget their future.
So if you really wanna live happily in you life, you will have to forget your past.
If you really wanna get something from your past that is experience. What all of mistakes you did in past.Learn frome your past and enter in your future this is best way to live happily.