This doesn't mean you just sit whenever the whim strikes you. It means you set up a practice schedule and keep to it with a gentle, patient tenacity. Setting up a schedule acts as an encouragement. If, however, you find that your schedule has ceased to be an encouragement and become a burden, then something is wrong. Meditation is not a duty, nor an obligation.
Meditation is psychological activity.
You will be dealing with the raw stuff of feelings and emotions. Consequently, it is an activity which is very sensitive to the attitude with which you approach each session. What you expect is what you are most likely to get. Your practice will therefore go best when you are looking forward to sitting. If you sit down expecting grinding drudgery, that is probably what will occur.