The first duty of all parents is to protect their children. Optimally, the parents will agree on this and negotiate in good faith for the best interest of their children Slow, deliberate, thoughtfully considered positions and explanations will generally get the best result in such cases.
That said, sometimes a parent is just being a jackass looking out only for themselves and the matter needs to be taken to the judge for resolution. Then you have to fight with all you have because nothing else will really matter if you ruin your children's lives by failing to guard their interests.
Any couple, no matter how much they detest each other or still like each other can end up in either situation. Knowing the difference is important and having an attorney with over a decade of custody litigation experience all over the state will make a huge difference no matter which situation you find yourself in.
Hopefully you can agree regarding your children, but if you can't, you know who to call.