Here's why:
* Intransitive verbs express actions or states that do not involve a direct object. They describe actions that are complete within themselves, without affecting anything else.
* Direct objects are the receivers of the action performed by the verb.
Examples:
* Intransitive: The bird sings. (The singing action is complete within itself. There's no receiver of the singing.)
* Transitive: The girl throws the ball. (The verb "throws" has a direct object "ball" which receives the action of throwing.)
Think of it this way:
* Intransitive verbs are like a solo act. The verb is the star, and there's no one else involved in the action.
* Transitive verbs are like a duet. The verb performs an action on a direct object (the other performer).