Pressure to have well-behaved children (always polite, always grateful, always cheerful) produces children who suppress authentic response. Some difficult behaviour — sulking, asserting opinions, refusing — is normal development of self-respect and individual personality.
What 'difficult' includes that's actually healthy
Strong preferences. Disagreement with adults. Bad moods occasionally. Independent decision-making that doesn't match parents. Not being immediately friendly with strangers.
Children who never push back become adults who can't either. Allow some difficult moments; address actual problems (aggression, persistent rudeness) specifically.