Imagine God Come down from heaven, holds your hand with the hand of an opposite sex and then announces to everybody on earth that, "This two have become husband and wife today; I have joined them together."

As thrilling as it may sound, that does not mean the marriage will be successful. Yes!
Did I hear you say God can do anything? Yes He CAN but that doesn't mean He WILL.
If He will do that, then He can as well make all sinners repent of their sins today, accept Jesus Christ and then we all fly to heaven. There is the part of man!

A successful marriage is a continuous effort of both partners physically, financially, mentally, character wise and spiritually...

The part of "God says" is there for the couple to run to God when challenges come ( Challenges will surely come), and to seek Covenant with Him. When you allow God to choose for you, you can trust Him for the Success of the marriage and He will guide you through, "if you allow Him to lead though."
The journey of marriage is lined with so many challenges and then, both partners must learn how to deal with them as they come. And there comes responsibility taking. If one of them do not take his/her part of the responsibility then the marriage is ready to collapse, regardless of any "God said." If one's part of responsibility is neglected, then a recipe for marital failure is ready, waiting for others to be ripe and then prepare garnished failure in the end.

Love is responsibility and it is sacrificial!
You want your relationship to work? First, make sure you're with the right partner and then learn the act of responsibility taking. Do you understand better now?