Service of the heart refers to prayer (Taanis
A young man complained to R Mendel of Kotzk that he was unable to concentrate on prayer because he suffered from severe headaches. R Mendel said, What does the head have to do with prayer? Prayer is with the heart, not with the head.
Chassidic writings, especially Tanya, emphasize that the
intellect can dominate over emotions. Emotions are a source of energy, and can be directed toward desirable or undesirable goals. One can direct his anger toward other people, which is bad. Or one can direct his anger toward injustice, which is good. All emotions can be directed either positively or negatively.
Many emotions are self-centered in a way that is detrimental to spirituality. The intellect must be put to work to turn the emotions away from physical desires toward spiritual concerns. Prayer, however, needs very little direction. As children we ask our parents to provide for our needs. Although praying to G-d for ones needs may be thought of as self-centered, this is
the one time when self-centeredness is not blameworthy. To the contrary, it indicates that one realizes he is dependent on G-d for everything. Praying for what one wants testifies that one believes in G-d.
Since the natural emotion of praying for something is commendable,
it does not need to be redirected by the intellect. One can pray from the heart.
But is it not necessary to use ones intellect so that one does not pray for the wrong thing? Do not worry too much about that. A child may ask for ice cream before dinner and even cry for it. A wise parent will not give the child something that is not good for him. G-d will not give you anything that is detrimental to you, regardless of how much you plead for it. We attest to this in our prayer for the new month: Fulfill the prayers of our hearts when they are for the good.
So pray. Pray from the heart.