Labor itself is a consumption (of energy) and thus a disutility. Labor acts on its input to transform them into something more valuable by mixing with them its 'mental' and 'physical' energy. Whether labor generates wealth or not depends on:
1) Luck -- Because of uncertainity of future, what labor has "produced" (transformation of inputs) may not be ultimately valuable. Good example, is burnt food.
2) Application of consciousness towards the goal -- Food might get burnt during cooking if one is not careful to continuously monitor it. This is function of will power. Ultimately, only thing that a "human" labor can really contribute is application of will towards a certain goal.