First, doing a startup is a really enjoyable form of technical work. You're working on what you want to, and that is fun in itself. It's also multi-disciplinary and thus you get variety and some quite arty bits of work. For example, How can we ask a question that will get us honest, interesting, succinct information about a user's personality?
Second, if your passion can't be your job (for whatever reason), make fucking sure you indulge your passion in your free time. I found that working a job that sucked hard to be an excellent motivation for making full use of my spare time by throwing myself into playing in a band, political activism, squash and programming my own stuff. If you don't get similarly motivated, perhaps forcing yourself to work on your passion will help.