Adults who seek their parents approval want to feel secure. Their parents had made them feel insecure in the past and gave them messages that taught them to not fully trust their decisions without deferring to their parents. These parents didn’t effectively parent, and treating their children like this harmed and stunted them.
Good parents typically approve of all reasonable choices their adult children make. If they don’t approve, they should at least accept letting their children be who they are and do what they choose.
Seeking approval from a parent comes with the desire to want them to be proud of you. It happens when the relationship with the parent has not been a really healthy one or the adult child would feel secure in themselves and not in need of approval.