הלכה על ברכות 56:13

As a result of the emphasis on the collective and fixed aspects of prayer, the personal component, with its intense and fervent passion, is liable to be pushed aside. This is the meaning of R. Shimon’s warning: “Do not make your prayer fixed, but [make it] a plea for mercy and supplication before God”(Avot 2:13). Rabbi Eliezer similarly said: “If one renders his prayer fixed, his prayer is not supplication” (Berakhot 28b).