The Midrash explains this with following illustrations: When the storekeeper wants to convince a potential customer that a garment is strong and will last, he will stretch and pull it. In this way, he shows the client that the garment is good. However, if the fabric is weak, the seller will not dare stretch it, for it may tear. Similarly, Hashem only tests people who can withstand them. This is why Hashem tests the righteous.