Not all couples who get divorced are inherently unhappy. Not all of them argue constantly. Abuse isn't always an issue. From the outside, they appear to be happy and well-adjusted, and most of their friends and family members would be shocked to hear that they're thinking about divorce.
Co-parenting, or working with your ex to parent your children after a divorce, can get very complicated. The two of you no longer live together and you may not be on good terms. All the same, you need to work together, communicate and coordinate your lives. Doing so successfully is in your children's best interests, and it's a necessity if you have court-ordered joint custody.