If you've formed a habit, it means that the reward happened consistently, which means you have high expectancy.

Just a sidenote: habits where rewards happen inconsistently maybe take more time to learn, but are also more difficult to rid of. If something was rewarded consistently, and you stop the reward, you broke the pattern. If something was rewarded randomly, and you stop the rerward, you didn't break the pattern, because "sometimes not receiving reward" was also part of the pattern.

As an example, procrastinating on web is addictive not because every hyperlink you click leads you to a precious jewel. It is addictive because of its random nature; perhaps the last ten links you clicked were an obvious waste of time, but who knows, maybe the eleventh link will show something that is really worth reading... so you click the eleventh link too. The randomness of reward makes it more exciting.