Tefillah is one of the four things that needs constant encouragement (Brachos 32B). The Chasam Sofer zy"a is said to have compared tefillah to a thermometer. Through prayer, one can measure the "temperature" of his Yiddishkeit. If one davens with enthusiasm, it is a sign that his Yiddishkeit is warm and fiery. If, however, one davens by rote, it is a sign that his Yiddishkeit is cold and needs strengthening.
A man once came to the Satmar Rov zy"a and related that a bochur had been suggested as a shidduch for his daughter but he had been unable to obtain accurate information, as some of the people he asked greatly praised the boy, while others weren't as complimentary. He said that he didn't know whom to believe. The Rov told him, "Go see how he davens. From the way he davens, you'll be able to tell what type of Yid he really is."
He used this idea to explain the pasuk (Mishlei 27:21): "A man according to his praise." How can one determine what type of person a man is? By seeing how warmly and enthusiastically he davens and praises Hashem.
