Although, before going on, it should be mentioned that dreaming about it because you do it so often is a very valid reason for dreaming of brushing your teeth. Our minds are connected to the things that we do on a daily basis, and our dreams are directly connected to behavior that we repeat fairly often, so if you are having a dream in which you are continually brushing your teeth, this might be because it is something you do so often. You probably brushed your teeth before you went to sleep and it is why you were able to draw up the memory so easily.
Your teeth are slowly decaying throughout your entire life. Some people are born with freakishly strong teeth, but for the most part, your teeth will always be in a slow perpetual state of decay. Brushing your teeth is merely a way of slowing down their decay. Even if you refuse to eat, which is the chief way that your teeth have that kind of damage caused to them, your gums will eventually shrink until there is nothing to hold your teeth in and they will fall out. No matter what you do, there is really no way of winning. You will simply have to watch your teeth decay away and try to brush them to slow down the process.
To dream of brushing your teeth, taking all of the above into account, means that you are trying to stop something from being damaged in your dreams, but are doing so in vain. You are trying to keep something together with full knowledge that it is already dissolving beyond your control. You cannot take care of yourself anymore. At least this might be one thing that the teeth symbolize, your personal health.
Another thing that your teeth might symbolize in the dream when you are brushing teeth is a relationship that you have with someone. You are doing all that you can to keep your relationship from falling apart, but no matter what you do, you recognize the signs that the relationship is collapsing around you despite all of the work you are putting into it. Sometimes in a situation like this you are just going to have to accept that your relationship is decaying just as your teeth are and you will simply have to let it go. It may have been fun when it was at its highest point, but the fact that it is failing no matter what you try should be evidence enough that you are not going to be able to salvage anything out of this relationship. It is over, and the sooner you accept that, the sooner you can move on.