What is the secret of a happy relationship?

There's no eternal happiness in any of worldly matters. Get married and start compromising. There are ups and downs, that's life. Hold up to your deen, always ask Allah for guidance and add the element of success (means Quran) in your life. There's not any easy answer to your question. Remember one very important thing; if you are in a haram relationship, like having a girlfriend or boyfriend, then remember that you can't ask for Allah's help into those matters, because He doesn't support the haram. That's the reason why most of such relationships don't end well. The relations whose base is immoral and forbidden, can't grow at all. Hence add the elements of purity and moralistic values in your relations. May Allah guide and help us all. Ameen.