Why practice asceticism? In Taoism, any external thing which holds your attention saps your life force or energy. I imagine this may be because when your attention is directed externally you aren't being "mindful" of yourself. The less mindful of yourself you are, the weaker you become. There are certain exceptions to this though. Some external things force your attention to move back inwards, creating a loop of sorts. One of the most obvious examples of this would be studying a holy or religious text with an appreciation of how it applies to you; usually treating things as being about yourself is a bad idea (because external things usually aren't about you, even if they want your money or will do anything to have your attention) but if the external thing is about you while also encouraging you to discard your individual ego then a positive feedback loop is sometimes created between the self and the external thing. This might also be related to the "path of action" and the "path of contemplation". Sometimes we engage in actions that can permanently change us for the better because the action engages or reveals something inside of ourselves, or the efforts of contemplation (sometimes facilitated with a "feedback loop" as described above) can give us new insight into ourselves.
