To answer, Jesus quoted Deuteronomy 6:5 and then Leviticus 19:18. The terms "heart," "soul," and "mind" are not completely distinct, watertight categories. They overlap somewhat and together cover the whole person. Taken together the meaning is that we should love God preeminently and unreservedly. [source][source][source]
"Jesus loves God with his whole heart, for he is blameless in his fealty to God ( Matthew 4:1-11). Jesus loves God with his whole soul, for he is prepared to surrender his life should God so will ( Matthew 26:36-46). And Jesus loves God with his whole mind, for he lays claim for himself neither to the prerogatives of worldly power [1] nor to the security of family, home, and possessions ( Matthew 8:20; Matthew 12:50)." [2][source]
The "and" in Matthew 22:38 is explicative. The one command is great because it is primary. [source][source][source]
The second greatest command is similar to the first in character and quality ( Matthew 22:39). It also deals with love (cf. 1 Corinthians 12:13). We should love our fellowman unselfishly (cf. 1 John 3:17-18). [source][source][source]
"A simple reading of Leviticus 19:18 . . . divulges that the command pertained to loving others, not oneself. The "as yourself" part of the command only furnishes a comparison of how Jesus" disciples are to love others." [1]7 [source]
The writer just quoted went on to discuss why it is inappropriate hermeneutically to argue from this command that one needs to learn to love himself or herself before he or she can love someone else. [source][source][source]