The answer to this question is subjective. Of course there are obvious examples of good and bad within music (a beginning student playing next to an adult professional). In this discussion we are comparing the various musical works of professional artists. To an individual, good is what they find musically and esthetically pleasing. In the music industry, good is what the popular majority finds pleasing or entertaining.
It is difficult to get the majority to agree. Peer pressure plays a role in this process. Often times people are afraid to step out and claim something as being good. What if others disagree with them? Many people will wait for the word of others who are considered to be experts in that field. If they say that it is good, it must be good. Other people may wait for a large group of people to label something good. If that many people think it is good, it must be good.
Unfortunately, the main way for us to have our music labeled as good (after you've practiced and done everything possible to make it sound its best) is to keep getting it out there for people to hear. In this process, many people will label it as bad. We will probably be rejected by most of the opportunities we pursue. However, all we need is that one opportunity that begins to change the tide. One person who is an expert in the field and believes in us. One group of people who enjoy our music enough that they are willing to promote it to their friends and relatives.
Getting through all the rejection to that point of acceptance is a long, frustrating and difficult path. Many musicians give up along the way. Others lose their motivation and settle for a life of mediocre practice habits and gigs. Some keep pushing through and never actually make it to the level that they are aiming for. If you really love music, though, it doesn't matter if you make it or not. You will keep writing, playing and singing even if no one wants to listen, because music is a part of you. You just can't give it up. So keep pushing forward and maybe one day you will make it.