Managing Wealth - An Issue of the Heart

In Matthew 6:19-24, Jesus begins by talking about wealth and where we should and should not store it. He then says something strange about the eye being the lamp of the body and its connection to light and darkness, which seems to be a shift away from the original topic of wealth. He then returns to the subject and says that you cannot serve God and mammon. Did Jesus veer off-topic, or is there an important connection we need to understand in this text?