The deep root of failure in our lives is to think, 'Oh how useless and powerless I am.' It is essential to think strongly and forcefully, 'I can do it,' without boasting or fretting.
Material objects give rise to physical happiness, while spiritual development gives rise to mental happiness. Since we experience both physical and mental happiness, we need both material and spiritual development. This is why, for our own good and that of society we need to balance material progress with inner development.