3 ideas
Inheritance binds subclasses to their parent's implementation at compile time, exposing internals and making changes ripple through the hierarchy. Composing objects from interfaces keeps each piece encapsulated and swappable at runtime, so behavior can be assembled and reconfigured without rewriting class trees.
An object maintaining state notifies a list of registered dependents whenever it changes, without knowing who they are or what they do. This lets you add or remove reacting components freely, since the subject only broadcasts updates and never references concrete observers.
Client code should depend only on an abstract type that declares what operations exist, never on the concrete class that performs them. This means any object satisfying the interface can be substituted, so dependencies are defined by capability rather than by specific implementation.