Haganos HaRav 67 (Soloveitchik RH Machzor):
The Rav objected in general to the introduction into the service of new prayers not included in the traditional liturgy, noting that Chazal, well aware of the paradox inherent in insignificant man approaching God to pray for his comparatively trivial needs, were thus determined to confine the performance of prayer to rigid, standardized texts based upon Biblical sources (Community, Covenant, and Commitment, p. 115)
He stressed that man’s entire right to pray to God for anything is based upon the fact that the Bible is replete with examples of people praying and petitioning god for various needs, and their actions therefore serve as a precedent for us (Divrei Hashkafah, p. 122). Consequently, no ordinary person can have temerity to compose his own formula of prayer, given that he lacks the necessary רוח הקודש, the Divine inspiration, which the Biblical figures had. The Rav further explained that it is for this reason that the Gemara in Megillah (17a) points out that the אנשי כנסת הגדולה, the Men of the
Great Assembly, who established the texts of our prayers and blessings (see Berachos 33a), included several prophets, because a level of Divine inspiration is necessary in order to properly formulate prayers. The Rav stated that he was unimpressed with various prayer texts composed by contemporary authors (MiPeninei HaRav, pp. 127-128).
Indeed, he held that no contemporary author has all the qualities that are indispensable for writing prayers. Nobody today has the inner ability, the depth, the breadth of experience, and the purity of soul that would authorize him to compose a prayer (The Lord is Righteous, pp. 298-299.)