Theophylact of Ohrid

Then, since not everyone is robbed, He adds an even greater and completely irrefutable reason. "For where your treasure is, there will your heart be also." Let it be so, He says, that neither moth devours nor thief approaches, but what punishment does the very enslavement of the heart to treasure buried in the earth and the casting down to earth of the godlike substance of the soul deserve? Is not the punishment all the greater for the one who possesses a mind? Where your treasure is, there will your heart be also. …

Commentary on Luke ↗

Augustine of Hippo

If you lack earthly riches, do not seek them in the world by evil deeds. If they fall to your lot, let them be stored up in heaven by good works. A manly Christian soul should neither be overjoyed at acquiring them nor cast down when they are gone. Let us instead reflect on what the Lord says: "Where thy treasure is, there your heart will be also." Surely when we hear that we should lift up our hearts, the familiar answer that we make should not be a lie.

Letter 189 ↗

Bede

For where your treasure is, there will your heart be also. This is to be understood not only about money, but about all passions. The glutton's god is the belly. Therefore, there he has his heart where his treasure is. The luxurious man's treasures are feasts, the wanton's amusements, the lover's lust, hence each one serves from whom he is conquered.

On the Gospel of Luke ↗

Bonaventure

Now great effort must be made regarding where the treasure is placed, because the mind is also placed in the same place; therefore he adds: For where your treasure is, there will your heart be also. Treasure is that which the mind principally loves, according to that passage in Matthew 13: "The kingdom of heaven is like a treasure hidden" etc. …

Commentary on Luke, Chapter 12 ↗