This is because you approach meditation as doing. Your samsaric mind is in the habit of locking in on a goal and taking steps to attain it. In those cases, the goal is something desirable, something valuable, something that should be good to attain. So when you are working towards a goal, you feel good about pursuing a worthy goal and you feel good about making progress towards something valuable.
However, meditation is not "doing". Meditation is "no-doing" or "un-doing" (wu-wei). The object of meditation is letting go. So for your samsaric mind, the goal of meditation is not valuable! There is nothing lucrative for it to lock on. This is why it feels demotivated. The objective of meditation is to kill the samsaric mind. Of course it has not a slightest interest in going in that direction!
There are two ways to solve this problem.
One is to set some kind of goal for the samsaric mind. Like Enlightenment or Nirvana. The more you think about the pointlessness of Samsara and the more you convince yourself how great and awesome it is to be Enlightened, the more motivation you will create for the samsaric mind. This is a good and valid method, especially for beginners.
For more advanced practitioners it may not work though. Because at some point your realization of Emptiness starts to mature and you may not be able to convince yourself that Nirvana or Enlightenment is a THING solid enough to strive for. At some point you begin to see emptiness of it all, and the tricks that work for a more primitive mind no longer work.
When you reach this point you should switch gears and approach meditation as an exercise of wu-wei, not doing. To do this you should focus exactly on being self-sufficient in the moment, on being in peace, on having just enough - without thinking of meditation as something you DO. Makes sense?
When you do it like that, what you practice is suchness which is the real point of meditation. Then, motivation is not an issue, or more properly speaking motivation is not applicable, because strictly speaking you are no longer "meditating". You are just dwelling in peace. Motivation is only needed for going after something. You don't need motivation in order to dwell in peace.