The Arizal writes that before saying one’s prayers, one should accept upon himself the mitzvah to love one’s neighbor as oneself “thereby his prayers will join those of all of Israel, rise up to Heaven and work effects.”

“If G-d forbid, the hearts of Jews are not united, neither will there be unity in Heaven.  When our hearts unite in the lower world, our souls unite in the higher worlds.  Thereby, our prayers too, join together, an Hashem accepts them.” ~ Kitzur Shulchan Aruch 12:2