One common misconception people have about child custody is that it is simple and easy to figure out. They plan to divide the time in half and split the responsibilities.
While that's an honorable intention and a solid goal, the reality is that these situations are very complex. They cause a lot of disagreements, even between parents who both mean well.
For instance, what school should the child go to? That sounds like an easy enough question, but what if one school is closer to one parent and works better with his or her work schedule, while another school provides those benefits to the other parent? You're both busy. You and your ex will both want the option that makes your life easiest, and there's no way to compromise here. You have to pick a school, and someone is going to be unhappy no matter what choice gets made.
Another important question is what after-school activities the child should participate in. Perhaps one parent played football in high school and wants to allow the child to play. The other parent never played sports and has read a lot about head injuries in football, and that parent is staunchly against allowing the child to play. Suddenly, there's a serious disagreement that the parenting plan has to address. Again, the result is not going to make both parents happy.
These are just two examples of minor issues; you can imagine how heated things get with issues like religion, medical care, vaccines and much more. Make sure you know your rights as a parent and what options you have.
Source: Live About, "Parenting Disagreements," Debrina Washington, accessed May 18, 2018