In the desert the Israelites grumbled. They were thirsty, irritable to be eating the same thing everyday. But they were doing this under their breaths or in their groups. The lord never denied they needed or did not need theses things, but what was more abhorrent was that they had no confidence or relationship with God, to state this to Him directly.
Contrast this with Thomas, he had doubts, and he expressed them freely, and he did not runaway from the situation. And Jesus blesses Him with appearing to him, and taking away his doubts.
Don’t grumble is today’s message then, don’t despair by yourself. Don’t give up hope and give up on God. The key here is – by yourself, Come and do it before God. Today is Divine Mercy Sunday – He who is Mercy, will bless you and show Himself (Mercy) to you.