Question

why are marriage are called that they are made in heaven

Answer 1

It is said that Marriages are made in heaven because it's not just a simple function where one marry another. Nope not at all.
It's joining off two minds and hearts. It needs understanding, forgiveness, and mostly no ego between each other. 
It's a beautiful relationship and one comes into our life and is going to be with us life long through our ups and downs.
And in this case I would say that marriages are made in heaven.
But still some may not agree because still there are many cases like misunderstandings, divorce ...
But a true couple's marriage is made in heaven I would say!
Hope this helps you Gowtham