When one meditates on a thought for a sustained period of time, beyond the moment in which it arose in one's mind, it becomes tangible and real.
That passing idea then becomes a meaningful thought that can build a complete structure.
Concentration and memory are like a song repeating its refrain within the words of prayer.
If a person has a fleeting thought of love or fear of God, and he takes that thought and sings it, repeats it again and again until he makes it a rhapsody, a tapestry, then he has turned an initial spark into a fervent flame.
The power in this way of prayer lies in continuity.
We engage in the work of prayer by taking a topic and working on it–not at random, not by happenstance, but together with the prayer–guiding the prayer in a specific direction over a period of time, whether that be an hour, a week, or a month.
Within that time frame and within those words of prayer, this topic is woven into the words of prayer, and it not only touches but is carved into one's soul.
Slowly, as truth is carved free of illusion–that which is permanent is carved from that which is transitory–one no longer has simply a single point of emotion and fervor, but a line that leads one from one rung to the next on the ladder of prayer.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose, Ch. 11 "Prayer," by Rabbi Adin Steinsaltz