From In the Way You Wish to Go (#499) by Rabbi Avigdor Miller
Q: Why are some prayers (like the verse, “Hashem tzva’ois”) repeated during services?
A: Repetition is one of the great forms of self-discipline. If you want to succeed in any ideal, it’s necessary to repeat that ideal to yourself many times.
Yetzias Mitzrayim (the exodus from Egypt) is reiterated many times in the tefilla (prayer) because of its importance; we have to remind ourselves about it always. Also, we have to remind ourselves always that “Hashem melech — Hashem is King”: He is the king, the Ruler of the world, and He conducts all the affairs of the universe with the greatest wisdom and kindliness. That’s an axiom of our existence in this world. Therefore it’s important at all times to remind ourselves that “Hashem melech.” It’s a pity that some people wait for Rosh Hashana to say that…
By constantly reiterating an idea to yourself, you’re training yourself
Any good thing, you have to repeat to yourself… When it rains, say, how good that rain is, what a pleasure it is when it rains! When the wind blows, say, what a pleasure it is! What a taanug (enjoyment) when the wind is blowing! When the sun shines, say constantly to yourself, “baruch atah yotzer ham’oros — Blessed are You who formed the luminaries.” That’s not only for when you sit to pray (when they’re praying, most people don’t know what they’re saying at all).
When the sun is shining brightly, especially these days, when the sun becomes stronger and stronger, you must remind yourself constantly what a blessing it is: “l’osei orim gedolim, ki l’olam chasdo — Who made the great luminaries, for His kindness is eternal.” How great is His kindliness in giving us sun and sunlight! You learn how to enjoy the sunlight. It sounds meshuga (crazy), doesn’t it? But if you choose happiness, then by reminding yourself and constantly reiterating to yourself, you’re training yourself, you’re teaching yourself.
The same is true with children. You should always repeat these ideals to children. You can’t always say it in the same words, because the child will be bored, so repeat the same idea with different words. After a while the child will finally get the message.