1. It is fitting for a person to stand up to pray when in awe and deference, as it says “Serve Hashem with fear”. (Shulchan Aruch and Mishne Berurah 93)
2. Even one who is unable to put awe and deference into his heart must at least make sure that he does not pray when light-hearted, or after having been involved in excitement or wasteful activities. He should also not pray when angry. Instead, one should pray when happy from having performed the commandments of the Torah. This is why we say the chapter beginning “Tehilah LeDavid” before some of the prayers, as it contains the verse “He fulfils the will of those who fear Him…” and “Hashem protects all of those who love Him…”. (ibid)
