At a deeper level, these words should be seen as G-d telling the Jewish people how to be happy at all times and in every situation.
First, “Be happy with all the good.” Ponder all the good that you have: the ability to see, hear, speak, a bed on which to sleep, clothes to wear, and other essential needs. Then you will discover that whatever is depressing you is only a very small part in relation to all the wonderful things that you have. Should you become miserable because of that small thing you don’t have? This is why the Torah emphasizes: “Be happy with all the good” — focus on what you have, and not on what you don’t.
You might ask: But what can I do if some things are making me miserable? So the Torah continues “that the L-rd your G-d has given you.” Remember that the One giving you everything you have is the L-rd your G-d — your Creator who loves you. Why would G-d, Who loves you and is able to do and give anything He wants, not give you what you need? There is no doubt that it is because His wisdom and His understanding is far beyond our own, and He knows better than us what is good for us. Whatever we think we are lacking is just our imagination, and we should trust that the Almighty will give us everything we need.
Our sages teach us that suffering is necessary and beneficial. Why is that?
Because suffering cleanses a person of his sins and endows him with a more truthful and sublime perspective of life, it is infinitely good for us. Sometimes what seems to us as “suffering” can lead to a great, amazing achievement and a much better situation than what we have now, and maybe even turns out to be a blessing!
This perspective will help one to understand that nothing that happens to him is “bad”, and everything is good. This is explained in the Holy Zohar (Breishit, page 150b) on the verse, “A Psalm of David when he fled from his son Absalom.” The Zohar asks why was David singing a psalm? When a man’s son rises against him to kill him, it is a far worse tragedy than when a stranger wants to kill him! The verse should have said “a Dirge of David”! David was able to sing a song because he realized that God wanted to erase his sin by giving him these torments while he was still in This World, so he was happy!
Another important message is included in the words “that the L-rd your G-d has given you.” Think not only about all the good you are enjoying but about the One who gave it to you. A person who gets a silver trophy from the king, exults more about the fact that he received it from the king than about its monetary value. He will tell everyone excitedly, “I got this from the king!” So how can we not be happy when everything that we have came from the L-rd our G-d?
This elevated state of mind is the lot of a person who looks at life and all the good he has in the right perspective.
Indeed, it is stated in Psalms: “Serve G-d with joy” (b’simchah). B’simchah has the same letters as machshavah — “thought.” Even though joy is an emotion and thought is a cognitive function, both of them are composed of the same letters. We learn from this that true joy is achieved by the proper mindset.