Is merely the entropy of the creative mind. Moreso found in comedy and music. Most people eventually run out of new tricks and tantalisations, alongside committed fans exposing themselves to all their material will inevitably find the cracks and repetitions. Especially when starting strong it's like gravity where the only way is down if you don't put the effort into feeding the brain with inspirations. The greats seem to be able to save their best till last. Which is my excuse for being so shit right now: keeping the good stuff to leave on, hehe 
Also have a half-arsed theory that getting married and saddled with kids generally dulls the fire within, or any other equivalent of attaining philosophical contentment, dare I say even happiness. In music it happens so much where their music loses bite once they get family stuff on the go. If you have a favourite artist I would recommend making sure they stay alone and miserable, it's the only way to keep that fine material flowing!

Also have a half-arsed theory that getting married and saddled with kids generally dulls the fire within, or any other equivalent of attaining philosophical contentment, dare I say even happiness. In music it happens so much where their music loses bite once they get family stuff on the go. If you have a favourite artist I would recommend making sure they stay alone and miserable, it's the only way to keep that fine material flowing!