Jesus knows how quickly our passions, even the most noble, can wear us out if we're not careful. I think he also understands that the sources of our exhaustion are many and complex and often we are completely unaware of how they are taking their toll. There are the obvious sources of exhaustion like a heavy workload, many family responsibilities, busy seasons when extra activity crowds in, but there are more subtle sources of inner exhaustion as well.
We might be functioning out of an inordinate sense of "ought and should," burdened by unrealistic expectations about what it means to be a good Christian. Since we're not always sure how to live with our humanness, we feel guilty when we are tired, ill or grieving and try to shove it down rather than attend to it. But it takes energy to repress these aspects of our humanness, and eventually the effort itself wears us out.
And Moses said to the people:
Fear not, stand firm, and see the salvation of the LORD, which he will work for you today...The LORD will fight for you, and you have only to cease, be still, be silent, and calm...for He is the LORD, your healer. (Exodus 14:13-14; 15:26b)