"Master of the world, Who reigned before anything was created" (Siddur).
The prayer Adon Olam is the opening prayer of the morning service; some congregations also recite it at the close of the evening service. It is also included in the extended version of the prayer upon retiring.
Adon Olam's being both the opening and closing prayer is similar to the practice of beginning the reading of Genesis on Simchas Torah immediately after concluding the last chapter of Deuteronomy. There, we indicate that Torah is infinite; like a circle, it has no beginning or end. So it is with prayer, which represents our relationship with God. Since God is infinite, we never reach a finite goal in relating to Him.
Indeed, the cyclical natures of prayer and Torah not only indicate that there is no end, but also that there is no beginning. Secular studies have levels of graduation which indicate that one has completed a certain level. In Torah studies, we do not complete anything. Indeed, each volume of the Talmud begins with page two rather than page one, to teach us that we have not even begun, let alone ever finish.
Growth in spirituality has no limits. The symbolism in the cyclical format of Torah and prayer is that we cannot say that we have even reached the halfway mark in spiritual growth, much less the end. This realization should excite us, not depress us, because our potential is infinite.