Phil Johnson over at Pyromaniacs has written a post titled "Why Slaves to Mamon Cannot Find Peace". You simply must read the whole thing but this line in particular was on the mark.
The key to Jesus' whole point is that last phrase. What you invest in is what you truly love. Where you put your treasure not only reflects where your heart is, but to a very large degree it determines what you think about, what you care about, and whom you serve. Invest your resources in earthly mammon, and you indenture yourself as a slave to a world-system that is hostile to God and exists under His condemnation.