“With all your heart,” those words carry a great depth of meaning, but as concerns the passage from Mark 12:30 it is easy enough to grasp the nature of what Jesus was speaking of—even if the actual living it out part is not so easy. It is my belief that when Jesus told his listeners to love God with all their heart he was essentially speaking of their desires, desires for God and Godliness.
He was instructing them that to truly love God with every aspect of their heart it must come from a place of willingness. And from the willingness and desire for God they would see a resulting change in character and compassion, passions and discipline.
I heard it said once that salvation was the process of changing our “want to”. It seems to me that loving God with all your heart has to come from that place inside us all. It is neither something that will happen by accident, nor can it be forced upon us. Rather it is the willing act of changing our desires.
Basically Jesus was saying, “want God.”