So, what would make you say "I have to draw that!" ?
Money.
Actually, the times when I've gotten completely obsessed about drawing have been because something isn't coming through properly - it can feel as if there is a blockage somewhere between my brain and the paper, and the actuality doesn't match expectations. Most of my pen and ink material isn't simply drawing, but a conversation with the subject, trying to work out exactly what I want. I love the way films sometimes show how artists are meant to have a concrete vision of the piece, then set about achieving that - it's not something I have ever experienced, and I'm not sure it is widely felt.