According to made-up statistics, 2 out of every 1 marriages end in divorce. Cosmically, the moment that you find happiness forces at least four other life forms into misery, and it's apparently unstoppable. Why so unhappy, everyone everywhere? It's hard to say. But all of the info on divorce kind of deals with the aftermath, and if After M*A*S*H taught us nothing, it's that what happens after is terrible in every way and doesn't leave anyone satisfied.
The lead up to this unhappiness is rarely discussed, and probably the reason for this is twofold -- habit and fear. If you love someone enough to commit to them, by and large that required an investment of time on your part. Maybe you met and got married on the same day, and that's awesomely irresponsible of you, but most people take it a little slower, and that slowness is what bends your brain over and asks it to drop trou while it reaches for something across the counter. You're used to this other person and, worse (in this situation), you're used to being in love with them. And that means it's going to be really hard to admit -- not just to yourself, but to them -- when you realize you don't love them anymore. Habit and fear, holding hands and dancing on your feelings, probably while laughing and smoking smooth unfiltered cigarettes.