Jesus is painting with a broad brush, using this statement.
Consider your time, money, affection and resources:
You invest/spend these things throughout the course of your life. Some things/people are bad investments ....some are good investments
(please don't misunderstand the meaning here...a "good investment" is one that yields a profitable return. I'm not saying that we should pick and chose who we preach to. This would be more relevant to discipleship and passing the mantle to faithful people who will steward their calling with diligence).
But whether you invest your life into something worthwhile or not, inevitably, you will be concerned with the things you spent your time/money/ etc on. People tend to care a lot about what they have made their life about. What Jesus said here explains why. When you disciple a person for several years, you tend to think about how they are doing more often than the average Joe in your life. When you spend time in the Word of God, it tends to become the main concern of your heart. When you give money to an orphanage in Ethiopia every month, you tend to think about them more often than the other orphanages around the world. When you impart your life to others, they become part of your life. If you go to many Sea-hawk games and buy their apparel, and decorate your house with their stuff, you will inevitably have a football team stored up in your heart, and they will occupy your thought life.
Jesus’ point was that if His followers had a treasure on earth they would think about it. But if they instead had a treasure in heaven, which is safe from theft and decay by moths, and were “rich toward God”, they would be concerned with matters pertaining to the kingdom and therefore would not be in a state of anxiety.
Seeking heavenly treasure means serving others and honoring God by doing so. If believers make the kingdom their consuming passion, then God will take care of other needs. Disciples, then, are not to fear but to trust God. They will not draw their security from possessions, and so they will be free to give their possessions to others. If their treasure is money, then that will be their consuming passion. Making money one’s treasure is the path to insecurity, however, because it is always subject to the vicissitudes of life.
If God does not own our possessions, then our possessions will own us.
What are we putting in our barns? We ought to invest in heavenly things, so that God is glorified and our hearts will be concerned with the things of heaven and not earth. If a person seeks such heavenly treasure, the implication is that his heart, i.e. his affections, is directed in the right way, whereas if a person piles up earthly treasure, the evidence shows that his affections are earthbound and hence his heart is not truly seeking God. This verse saying provides a motive for the preceding command, by showing that the person who continues to hold on to earthly wealth and does not fulfill the command in is not really seeking after the kingdom of God.
One cannot serve God and money, but one can serve God by the correct use of money.
Our heart attaches itself and is concerned with the things we invest our lives in. By definition "treasure" is something that is "stored up" or "held onto".