As an aside, this is how null is defined in many languages as well. Many behaviors of SQL, for instance, are much more intuitive if you remember that SQL sees null as "unknown".

If a boolean value is null in SQL, it conceptually means it could be either true or false - it's a maybe. If you ask "is maybe true equal to maybe true", the answer is "maybe". Likewise, asking SQL 'null = null", it answers "null", instead of true/false.