Whether plea or praise, prayer is always speech addressed to God, and such speech is only possible when a person knows that "Verily God has heard me and attended to the voice of my prayer" (Psalm 66:19).
This realization that "You hear the prayer of every mouth" is what directs man to pray and to confide in God all those secret, personal matters, needs, anxieties, requests, and heartfelt desires.
But in order to pray in this manner,to "pour out his complaint before the Lord" (Psalm 102:1), one needs to feel a sense of intimate closeness to God, as "Our Merciful Father."
The child standing before his father feels he can tell him everything in his heart: to plead, to complain,to thank, or to simply tell him about things.
–Rabbi Adin Steinsaltz
From A Guide to Jewish Prayer, p.10, by Rabbi Adin Steinsaltz