Do you fall out much? That might do it. There's little point in committing to anything you know right from the start won't be harmonious. It could also be something about you he's uncomfortable with or just that he doesn't see you as a life partner for him. On the positive side perhaps he takes marriage very seriously so if he does commit it will be 100%.
Because he's not ready to commit. Don't push that issue as making him make a decision may not be the healthiest for the relationship in general.
maybe you're giving him a reason to think after a year or so you'll become a **** and cheat on him (: