Sometimes a person's heart spontaneously opens, and he finds himself able to stand before God.
But such moments of inspiration are infrequent, indeed so rare that it was deemed necessary to designate specific times for prayer.
Without such a schedule, a person might seldom think of God.
But because we are bound by fixed prayer times, we are obligated to turn to God three times a day, and thus form some sort of daily relationship with the Almighty.
Although this meeting with God may be under duress, it is at least a point of regular contact that does not depend on one's mood, disposition, or inner preparation.
It is like setting a date, which depends on finding the time and the place.
And when it is set up, one comes.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose by Rabbi Adin Steinsaltz