Since tefillah is such powerful, good force, the yeitzer hara fights bitterly to obstruct it. Sefer M'Ohr Vashemesh (Parshas Ki Seitzei) writes that the yeitzer hara's main focus is on tefillah. While it also tries to stop a person from learning Torah, doing mitzvos, and performing acts of kindness, it works much harder against tefillah. It does this by assailing a person with foreign thoughts that distract him while he is trying to daven.
Therefore, one must try very hard to banish all outside thoughts while he is davening. If one succeeds in doing so, he reaches an extremely lofty level of almost completely stripping away his connection to the physical word (see Shulchan Aruch, Orech Chaim 98:1). The yeitzer hara does everything in its power to stop this from happening, so one must be constantly on guard.
