There are other religions beside Trinitarian theism that believe in one God. And you shall love the LORD your God with all your heart and with all your soul and with all your might. The first part of this passage is one of the great texts affirming the essential unity of God. And love is the proper human response to Him. This love is not some squishy feeling, but rather an expression of devotion from someone to someone.
Love has a source and love has an object.
Since human beings are created in the image of God, then He must be capable of love in His very self. A mono-personal God is not 'big enough' to be God. Simply stated, a mono-personal God cannot be both just and the justifier. And if God exercises justice against the sinner, then he has denied his mercy. For a mono-personal God, compassion contradicts holiness, forgiveness is finally contrary to justice.