Most Helpful Girl
It depends on their age. A 20 yr old will likely have different reasons than a 40yr old.
I've seen a number of people, who don't have direction, who were pressed so strongly by family (or community) that they never have the time to discover themselves and find what they want.
Some people who grow up in poverty or small towns never really believe that can reach for something more. They don't have a role model for anything different so they just follow the path everyone else has.
Another reason is codependence, the individual loses them self in their partner. Once the relationship ends they are left floating and lost.
Others are just flaky or haven't taken the time to experiment and find the right path. Having kids early unexpectedly can be a big one that derails a person's expectations.