Re: cavity between teeth
There are several classes of cavities. First you have to understand the three layers of your tooth. The outer layer is enamel, the middle is dentin, and the inner layer is the pulp. Where the enamel and the dentin meet, it is called the DEJ or dentin-enamel junction. If a cavity hasn't crossed the DEJ, then it is possible for the cavity to heal. This is called remineralization. If you can find a way to keep you mouth a little cleaner, use a little bit more fluoride, etc., then it is possible that the cavity will heal up and you won't need to have it filled. Once it crosses the DEJ on the x-ray, it usually needs to be filled. Lasers do the same thing as drills, they cut the tooth. They just cut it differently. It's like cutting down a tree with an axe or a chainsaw. Either way, it still gets cut. There are other details about lasers, but that is the bottom line.
In the end, if you don't think your habits are going to change, or you don't think SOMETHING is going to change, then waiting and watching means you are just going to wait and watch it get bigger! No sense in that. Waiting and watching implies that the dentist hopes it is going to get better, not bigger. If he thought it was going to get bigger, than it wouldn't make sense to wait.
Cavities can indeed get big in a hurry, but they can also take years to develop. Usually an dentist will check it again in 6-months. If this has developed very quickly, you might think differently than if it had been brewing for a couple of years.
Finally: the way it is fixed is usually that they drill down from the top of the tooth to get at the cavity in between the teeth. Then they fill the hole with a silver metal filling (amalgam) or a white tooth colored filling (composite). If you want to read about it you want to search for terms like "slot prep", "class 2 amalgam restoration" and "class 2 composite restoration". That should give you more information, and a few pictures.
Remember once, you cut a hole in a tooth, you can't ever put it back. So I suggest trying to figure out what you can do to help that cavity remineralize. Good luck to you.