There is both a short and long answer to this question, and I will give you both in case the longer explanation is helpful.
Short Answer: It is good because the person who values the good the most (has the highest demand) receives the good, and it is bad because this means the person with the most resources gets the good at the expense of others who may want it but don't have the means to afford it.
Long Answer: Demand can be though of as willingness to pay. For example, I really like ice cream. If there is one ice cream store in town, and the price of ice cream is based on an auction, I will pay the most because I have the highest demand, the highest willingness to pay. In economics this is a "good" outcome. But, maybe I am able to pay more because I have the most resources, and have had opportunities others around me may not have had. Others may demand, or value, ice cream more than me but not have the willingness to pay what I can. Thus it's a non-equitable, or "bad" outcome from a social perspective that I received the ice cream.