Prayer becomes trivial when ceasing to be an act in the soul. The essence of prayer is agada, inwardness. Yet it would be a tragic failure not to appreciate what the spirit of  halachah does for it, raising it from the level of the individual act to that of an eternal intercourse between the people Israel and God; from the level of an occasional experience to that of a permanent covenant. It is through halachah that we belong to God not occasionally, intermittently, but essentially, continually. Regularity of prayer is an expression of my belonging to an order, to the covenant between God and Israel, which remains valid regardless of whether I am conscious of it or not. Abraham Joshua Heschel

Quoted in Or Hadash: A Commentary on Siddur Sim Shalom for Shabbat and Festivals by Reuven Hammer, RA-USCJ 2003