This is a great question. The three basic points of St. Bernard's work, On Loving Godare as follows:
First, St. Bernard gives reasons why we should love God. Second, he exhorts Christians to love God. Third, he writes of the benefits of loving God. In light of these points, this work was written to help Christians love God more in a way that honors God, but also in a way that deepens the love of Christians for God. More specifically, On Loving God was written for Bernard by Haimeric.
Here are some quotations to give you a sense of the work:
"When one asks, Why should I love God? he may mean, What is lovely in God? or What shall I gain by loving God? In either case, the same sufficient cause of love exists, namely, God Himself. And first, of His title to our love."
Again he writes:
"Hence, if one seeks for God's claim upon our love here is the chiefest: Because He first loved us (I John 4:19)."
Here are the table of contents:
Why we should love God
How much God deserves love from man
What greater incentives Christians have to love God
Of those who find comfort in the recollection of God
Of the Christian's debt of love
A brief summary
Of love toward God not without reward
Of the first degree of love
Of the second and third degrees of love
Of the fourth degree of love
Of the attainment of this perfection of love only at the resurrection
Of love: out of a letter to the Carthusians
Of the law of self-will and desire, of slaves and hirelings
Of the law of the love of sons
Of the four degrees of love
As one can, see the work is very practical and written in a very simple style. It is practical manual to help people love the one who loved them first.