What happens when we execute devotion with motive?

Hare Krishna Prabhus and Matajis,
Please accept my humble obeisances! All glories to Srila Prabhupada and Gurudev!

Srila Prabhupada gives a nice explanation of devotional service and how it should be rendered in the purport of 4.9.17:

"Pure devotional service is without material form and is not covered by mental speculation or fruitive activities. Pure devotional service is therefore called ahaituki, unmotivated. Dhruva Maharaj knew that he had come to worship the Lord in devotional service with a motive - to get the kingdom of his father. Such an adulterated devotee can never see the Supreme Personality of Godhead face to face. He therefore felt very grateful for the causeless mercy of the Lord.

The Lord is so merciful that not only does He fulfill the desires of a devotee who is driven by ignorance and desires material benefit, but He also gives such a devotee all protection, just as a cow gives milk to the newly born calf. In the Bhagavad Gita it is said that the Lord gives intelligence to the constantly engaged devotee so that he may gradually approach the Lord without difficulty. A devotee must be very sincere in his devotional service; then, although there may be many things wrong on the devotee's part, Krsna will guide him and gradually elevate him to the highest position of devotional service." 

Your servant,
Devakinandana das