Luke 12:34-35 KJV
For where your treasure is, there will your heart be also.
Let your loins be girded about, and your lights burning;
The context takes us from covetousness through treasure to light.
Light has to do with our preparation for the Lord's return. The believer is to be a light in this world, declaring that he seeks a better country.
But we can not convincingly make that declaration if we are entangled in this world.
The proof that we look for the Lord's return is that we hold such little worth in this world.