Do you know the "Roly-Poly" toy? The one you push over, it falls, but it stands right back up on its own? It's like that because it has a firm weight inside. And a glass cup? If it falls once, it breaks and never stands up again.
The Bible says our "spirit" (what we feel inside) has to be like the Roly-Poly. If we get sick or sad (get pushed over), our joy in Jesus makes us stand back up again! But if we stay grumpy and without hope, we end up like broken glass on the floor.