Fear of Loneliness
Everyone has had a moment, at least once, where they picture themselves as an old man or woman who is all alone, no spouse, no children, no family. Some take it even farther and imagine they have no friends. That picture is terrifying. Humans are social creatures. Without connections, they suffer from a variety of disorders including depression. Thankfully, for most people, those terrible visions of living and dying alone are only temporary. They spend a moment or two shivering and then remind themselves that there are millions of fish in the sea. They will find the person that is right for them eventually.
Sadly, some people struggle to internalize the knowledge that there are other people out there who might be right for them. So, they marry whoever they are with when that fear strikes. Most often, this affects people at a transition point in their life. They marry their high school sweetheart, the person they are dating at the end of college or someone from their hometown right before they change states to start their career. In such cases, you are not emotionally ready for a marriage because you are reacting in fear. Your spouse deserves better than to be chosen simply because they were there when you panicked.