Reward
When you're working hard during the day, you would like to be looking forward to some kind of a reward in the evening. Something that you will enjoy. Something to balance out the hard work that you did during the day.
When there's nothing to look forward to, it's difficult to summon the will. Because, in this case, no matter how hard you work – there's gonna be no reward anyway. So why bother?
It doesn't mean that you can't enjoy the process of work itself. Moreover, it's highly desirable for you to like doing what you do.
Just make sure that there is a reward waiting for you on the other side. Hard work needs to be rewarded.