It largely depends on your definition of happy.
I think it also depends on what kind of a person you are. If you're restless spirit any relationship would become a jail with time...Or a place where you feel kind of trapped. If you had too high expectations at the very begining it might also dissappoint you on the long run...
I don't know. I'm married for ten years. I could never imagine myself divorce...I kind of know that there is no perfect match for me anywhere on this globe...I accept the fact that i can't have those butterflies in my stomach forever, or that infatuation that use to drive me nuts about him. He has a lot of iritating flaws and i can't say i enjoy them But i'm also very much aware that i have a tons of flaws myself. And he also chooses to stand by me anyway.
A marriage is made of everyday effort. Everyday trying to make things work...So there you go. If you have someone who is ready to take you in his arms when you're misserable or be kind to you at your worst...i think it definitely speaks it's own story. If your sex life exists, and you're satisfied with it, it's also a pretty good sign that you have some thick stuf, sticky matter that bonds you together-still...

Grass often tends to seem greener in a neighbours yard, but it's usually just an illusion.