Human beings don't ever grow up.
Whatever "grow up" means.

Thus in modernity, they get themselves into so-called romantic relationships.

Reserve the right and whims to behave like a child, and be parented.
Exercise logic and rationality to behave like an adult, and be a parent.

That's why couples hold hands and call each other "Baby".

The volatility of combined love and fear hence attains miraculous equilibrium.