"And they ask you how much they are to give, say: "The excess." It is thus that God clarifies for you the revelations that you may think." Qur'an 2:219
"O Children of Adam, dress nicely at every temple, and eat and drink and do not indulge; He does not like the indulgers." Qur'an 7:31
"And give the relative his due, and the poor, and the wayfarer; and do not waste excessively. Those who waste excessively are brothers to the devils, and the devil was to his Lord a rejecter." Qur'an 17:26-27
"And those who when they give they are not excessive nor stingy, but they are in a measure between that." Qur’an 25:67
"No, you are not generous to the orphan.
And you do not look to feeding the poor.
And you consume others inheritance, all with greed.
And you love money, a love that is excessive." Qur'an 89:17-20
The Qur'an stresses the importance of charity over and over again. It urges Muslims to be moderate in all things -- in eating and drinking, in work, in the family, in worship, in finances, and all other aspects of life. It warns us against consuming excessively, because that is greed and waste.
Islam is a faith of balance, where the individual, through moderation, is most fully able to reach his or her potential and live the best life they can.
This doesn't mean living in big houses, with flashy cars, expensive clothes, and the like.
Think about it: if you're spending all of your money on material things, how much is left to go to charity?
How can you help others when you're tied down to working too much to pay for that stuff?
How can your relationship with your partner/spouse and family be good when you're too tired to spend time with them?
How can you flourish spiritually when having too much around you is a cause of stress and distraction?
The answer to all of these questions is: You can't.
There is a direct correlation between the stuff in our lives (the distraction clutter causes, the constant need for cleaning, organizing, and maintaining) and spiritual fulfilment. The less stuff you have, the less time and effort and mental occupation it can take up. Instead you can devote that mental focus to developing your relationship with Allah (or whatever deity you believe in). That's why mosques are empty rooms -- it promotes both cleanliness (very important for prayer and worship) and lack of distraction.
Spiritual contentment is a big factor in over-all happiness for many people.
Just ask yourself: What matters more to me? This stuff? Or having the time and emotional/physical/mental energy to get closer to Allah?