There’s a traditional Jewish prayer called the “Ashrei.” It’s entitled after its 1st Hebrew word. “Ashrei” means “Happy [is …].” This prayer appears multiple times in the weekday, Shabbat and Holiday liturgy. The Talmud says, ”Who-ever recites [“Ashrei”] 3 times a day is assured of belonging to the World to Come.” 
It’s composed almost entirely of Psalm 145 , with two introductory verses borrowed from another psalm. Psalm 145 is an “alphabetic acrostic.” Each verse begins with a successive letter of the Hebrew alphabet, starting with the letter “alef [א] .” The introductory verses both begin with “alef,” as well, in keeping with the poetic form. 
The rabbis mandate that “Ashrei” be said mainly for the sake of one verse in it: the one beginning with letter “peh [פ].” “Po’tei’ach et ya’deh’chah u-mas-bee-yah l’chol chai ratzon:” “You [G-d] are opening Your Hand, and satisfying every living thing with favor [or: willingly].” We’d therefore be stating this three times daily. Of course, the rabbis intend that we mean what we say. They want the constant, sincere repetition of this statement to ultimately color our view of daily life itself, in a “positive” way. To accomplish this, they give us this verse as the quintessential affirmation of “bitachon” — trusting G-d.
To say “You [G-d] satisfy every living thing willingly…” is to say, “G-d, You’re taking care of me, graciously, at every moment.”
The connection between “happiness” and a belief in G-d’s infinite kindness and generosity is made clear enough in the Midrash and Aggadah. Why, then, do the rabbis require repeating this prayer at least 3 times each day? Why does doing so guarantee a “place in the World-to-Come?”
For that matter, why a liturgy at all?
Left to ourselves, we often search for words to say to G-d. Often, we unnecessarily “beg” of G-d a Divine generosity that is already always forthcoming. To paraphrase Ernest Holmes: G-d is already giving everything; do we really need to ask for more?
Liturgy is a “prepared script” of prayer.
Liturgy is meant to take the “searching for words” out of expressing ourselves in prayer. It’s meant to allow prayer to become something fluid; something that doesn’t require a separate, conscious effort for each word.
Think of typing: when we first learn, we have to think about how and where to move our fingers for each letter. If we do it regularly, we gradually develop a fluidity that allows “typing” to become a vehicle for deeper self-expression, with less conscious attention needed for each individual stroke.
Or, when an actor learns the words of a script, they’re still only “words,” no matter how well written, until the actor/actress uses them as vehicles for a shared experience with the audience of a play. But to do so, the actor must know the words well enough to be able to speak them as if they were his/her own.
Liturgical prayer is the same: an outer, fixed “script” that’s meant to become a vehicle for our own, fluid self-expression, through an intimate familiarity that’s developed by innumerable repetitions. In the process, it’s expected that we’ll be changed by the words we say, as well:
[Rebbe Nachman of Breslav said] “Let your heart hear what your mouth speaks, if you wish to offer proper prayer.” 
On a deeper level, though, liturgy is not so much about the outer words, as it is about the thoughts that accompany those words. If our prayer is sincere, our words and thoughts will be the same. If we say the words, but think of other things, we’re like actors on a stage, reciting the words of a script, with an affect that can be no more than superficial. The “audience” will hear it and feel it, just as G-d will “hear” and “feel” the degree of unity of our words and thoughts. But most serious of all – we know if the words we say are really meaningful to us, or whether they’re just something we’re “expected” to say.
Finally, liturgy is about allowing the conscious mind to get out of the way altogether — rather like the way the best Jazz musicians (among other types) ultimately allow the music to “come through” them. The Ba’al Shem Tov described it as reaching a point where you feel it’s the Shechina praying; not “you”:
“[When praying, the]…true worshipper [hasid?]…acts as the emissary of the Shechinah to bring thoughts into words.” 
It’s about allowing something spontaneous to happen inside you as you pray — “let go and let G-d.” The words of the prayers — their sequence; their meanings — and our familiarity with them are, in the end, for the purpose of transcending them completely.