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NULL indicates an absence of a value. The designers of SQL decided that it made sense that, when asked whether A (for which we do not know its value) and B (for which we do not know its value) are equal, the answer must be UNKNOWN - they might be equal, they might not be. We do not have adequate information to decide either way.
You might want to read up on Three valued logic - the possible results of any comparison in SQL are TRUE, FALSE and UNKNOWN (mysql treats UNKNOWN and NULL as synonymous. Not all RDBMSs do)
NULL is an unknown value. Therefore it makes little sense to judge NULL == NULL. That's like asking "is this unknown value equal to that unknown value" - no clue..
See why is null not equal to null false for a possibly better explaination
You cannot use = for NULL instead you can use IS NULL