Here's why:
* Transitive verbs require a direct object. This means the verb acts upon something.
* Intransitive verbs do not require a direct object.
Example:
* "The envelope encloses the letter."
* "Encloses" is the verb.
* "Letter" is the direct object. The action of enclosing is happening to the letter.
Therefore, "encloses" needs an object to complete its meaning, making it transitive.