In many dysfunctional homes, love is conditional and based on performance. The child learns that being "perfect" is the only way to avoid criticism or chaos.
Adulthood requires a foundation of security. When a child has to be the "adult" in the house—managing a parent’s sobriety or sibling’s safety—they lose the ability to be spontaneous.
The primary cause of the adult child syndrome is a home environment characterized by inconsistency. In homes where a parent struggles with addiction, mental illness, or extreme rigidity, the "rules" change daily.
Parents may use the child for emotional support (parentification), forcing the child to skip the "play" and "discovery" phases of youth.