Mahabba

"I planted a branch of desire for the people of love
And not one knew, before me, what desire was,

It sprouted branches, and sensual longing ripened
And left me with a bitter taste from the sweet fruits.

The desire of all the passionate lovers,
If they were to trace it, comes from that source.

- recited by Ibn Ata

God's love for the servant is His desire to bestow blessings specifically on a given servant, just as His mercy for Him is His more general desire to bestow blessings.

So mercy carries a more specific meaning than desire, and love carries a more specific meaning than mercy.

God's desire to extend rewards and blessings to the servant is called "mercy", and His desire to confer nearness and exalted state on him is called "Love".

(Source: Al-Qushayri; Principle of Sufism)