most people are subhealthy, and the competition is too intense, we feel much pressure from all aspects of the society, not only work, economy, but also life and so on.
you can relax yourself by travelling, singing, doing sport, reading and so on.
人类做任何事情,都必须付出能量,能量用完了,你就干不动了,这不是很正常吗?如果你一直都不会累,那你岂非成永动机了!!!!!!!!
总得来说,分生理和心理两个原因.(废话,哈哈.....