You Forgot Your Floaties

There are only two sources of magic, Madness and Sadness.  Any class can have any source or combination of sources.  Your source is more than your external emotions or actions.  A Magic User's source is the armature over which other feelings hang and take shape.

Madness

Merlin and the great druids are motivated by madness, a sense of destiny that they force onto a meaningless and random world.  They have looked into the void that lies at the heart of all things and denied it.  There must be more.  Their belief gives them power over the world, since the world is something they created to cover their fear.  How could they not control it?  But every time they cast, every time they summon some impossibility, they are reminded that below their facade is empty air over an eternal drop.

Harry saw the void under the stairs, and denied it

Sadness

Magic Man and some of the great bards are motivated by sadness, an abiding knowledge that dogs them.  The world is nothing, a phantom.  They looked into the void, and their illusions and denials died on their lips.  Then, the cruelest part. They had to continue living their life. It's easy to forget, in the tumult of everyday, what they know. They might find companionship or distraction desperately, not sure what it is they're running from.  When they are alone, when the songs have been sung and everyone else in in bed, their knowledge is waiting for them.