God told Israel that the whole nation should appear before him several time each year. These times were to celebrate special commemorations of who god was, and what God did for Israel.
One of these was at harvest time. Israel was supposed to give a portion of what was produced to God. As they gathered together, collecting the offerings, they were supposed to realize the huge work of provision God has blessed the nation with. The realization should produce joy.
Again, the heart is focused on God ... and a particular part of his nature. God is amazingly generous. He provides for each of his children's needs ... and often for their wants.
It is tempting for us to focus on the stuff itself. Joy will not last that way, because the stuff will not last. We need to look beyond the stuff to the giver.