The one major difference between God and the Devil is that the Devil is downright and madly in love with his job.
The one reason why the Devil is happier than God is that the Devil is downright and madly in love with his job, which for now we shall call 'his labour' and God is in love with the fruit of His labour, His creations - Us. (That is if we go by what almost all major religions say, 'God loves you!')
And in all His infinite wisdom, God once said what I will now paraphrase: "Love your work not the fruits you receive when you're done."
Oh, oh and before I forget, I have a theme song for this post: