Very good observation! It is true that in Deuteronomy 6:5 we are told to love the LORD with all of our heart, soul, and strength, and it is true that in the Gospels Jesus includes the “mind.” You may have also noticed in Deuteronomy the word all. We are not merely to feel nice things about God, but we are to love Him with every facet of who we are—heart, soul, and strength. The mind is clearly implied in this text, but Jesus highlights what was implied by adding the word mind. He emphasized the importance of the thought life and mental devotion of believers. He deliberately extended His own word to include the intellectual dimension of Christian living. We are to love the LORD with all of our minds, which entails learning how to think Christianly and biblically about all of life.
For help with loving the LORD with the mind, I suggest the following books: The Christian Mind by Harry Blamires and The Scandal of the Evangelical Mind by Mark Noll.