Monique named a good cause of marital breakdown...third parties. In my marriage, we had a rocky foundation to begin with, but when things started going awry, third parties looking for gossip and despair preyed on that and further caused the problems, and my ex and I let them. In the end, it's the fault of the couple for the lack of communication, respect, trust, and consideration, but third parties can further a shaky marriage to crumble considerably.
Marriages fail because we've romanticized how they should be in movies, books, and pictures. We aren't teaching the next generations how to effectively communicate, be respectful, how to fight fair, and what it means to truly love someone and be in a committed relationship, and you're seeing the effects of all that now. Marriage is work. It's fun, exciting, boring, contentment, tiring, depressing, relaxing, annoying, silly, connecting, and meant to be permanent, all in one package, and a lot of people think it will be all rays of sunshine for the rest of their lives, and when the realization sets in that their expectations were too much, it starts to eat away at their relationship in different forms.