(Kadamba Kanana Swami, November 2015, Vrindavan, India, Video Interview)
When the living being turns away from Krsna then suffering begins. Happiness only exists in the relationship with Krsna so when we turn away from Krsna, we begin to suffer. Then Krsna has also added a little suffering, he added some sand-in-the-sweet-rice. Why is this so? Because Krsna loves living beings dearly and Krsna is more eager for living beings to go back to the spiritual world than the living beings are themselves. Therefore Krsna cannot wait, so sometimes he puts a little sand-in-the-sweet-rice just so that when we take sense gratification, we come to the understanding that maybe sense gratification is not worth it – it will not let us go back to the spiritual world! So ultimately, the elevated liberated souls see even suffering as the mercy of Krsna.