Most spiritual traditions encourage simple living, and yoga is no exception. In the Yoga Sutra, Patanjali laid out the yamas (moral restraints) and niyamas (observances), a set of 10 principles that are crucial to one's progress along the yogic path. One of the yamas is aparigraha, often translated as "greedlessness."
The desire for external wealth causes unhappiness on both a practical level and a spiritual one. In order to afford things, you have to work long hours, leaving you less time for what truly sustains you, whether that's yoga and meditation, a hobby, or time with your kids. An expensive lifestyle also limits your choice of career, forcing you to take a high-paying job that may not be fulfilling. It's hard to transcend the desire for external things when we see hundreds of ads implying that happiness lies in a new iPod, laptop, or car. But despite those commercial messages, acquisition doesn't equal happiness. Many yogis find that if they transcend their material cravings, they can lead more satisfying, albeit more modest, lives.