“They are being tested by many troubles, and they are very poor. But they are also filled with abundant joy, which has overflowed in rich generosity.” (2 Corinthians 8:2 NLT).

Paul commended the churches of Macedonia for their amazing generosity. For although they were very poor, they were very joyful and insisted on giving generously to the offering Paul was collecting for Jerusalem.
God often chooses the poor to accomplish great things. For they readily depend on Him and easily give Him all the credit. When people gain even a little wealth, they tend to depend on it, rather than God. Of course, being poor does not automatically make one more spiritual. A poor person can be as grumpy and miserly as a rich one. Yet, riches of any amount, can become idolatrous to the one who depends on them.
Generosity is the mark of one who recognizes God as the source of all they have and depends on Him for all they need. Joy and generosity go together.