The Arizal writes that just as children are obligated to respect their parents, so must they respect their firstborn sibling. For the firstborn represents the initial creative act of the parents, from which all subsequent births draw their vitality. Thus, Jacob said about Reuben:
"Reuben, you are my firstborn, my might and the beginning of my strength". (Gen. 49:3)

Be very careful what you say [in the morning, before praying]. Our Sages were strict even regarding permissible words, such as greeting someone before prayer, because this can also cause a blemish. It is known that the world was created with thought, speech and action. The first level is thought. Speech is an offshoot of thought, and action an offshoot of speech.

If his first words are mundane...everything he says later will be influenced by them...

When a person rises each morning, he is also a new creation, as the verse says: "They are new every morning". (Lamentations 3:23) If his first words are mundane (and all the more so, if they are forbidden, such as profanity, gossip or slander), everything he says later will be influenced by them, even his prayers and Torah studies. Since speech follows from thought, so to the second word follows from the first.

This is similar to the teaching of the Zohar (3:83a) and the Arizal on the obligation of siblings to honor the firstborn. The firstborn takes the main portion, whereas all other siblings are like offshoots from him. In our case too, one must be very careful to sanctify and purify his first words and thoughts, and attach them to holiness, so that all subsequent words should follow them. Then, when he starts to pray, amidst the joy of having fulfilled the mitzvah of sanctifying his speech and thoughts, his words will surely be answered.

[Translation and Commentary by Rabbi Dr. Eliezer Shore from Kesser Shem Tov, 20b; first published on //besht.com.]