From The Gospel of Mark by Mary Healy, comnenting on Mark 12:29-30:
To love God is to have a profound reverence and affection for him, to give ourselves over to him and desire to please him above all else. Jesus is spelling out what he had said earlier about repaying to God what belongs to him (12:17).
Jesus uses four terms that, taken together, signify not distinct faculties or parts of the human being but different ways of referring to the whole person. The heart (kardia) is the inner depths of a person, the wellspring from which all our decisions and actions flow (see 7:19). The soul (psychē) is our whole self as a living being, that which Jesus said we must be willing to give up for his sake (8:35) and which he will give up for our sake (10:45). Jesus adds another term, mind, to emphasize that even our thoughts and reasoning must be animated by love for God. The last phrase, with all your strength, emphasizes that love for God is not a sentiment that arises spontaneously, but a commitment that calls for every ounce of our energy. How can such love without measure be possible? Only by our first knowing and experiencing God’s love for us (Rom 5:5, 8; 1 John 4:11).