NEVER DIVE ALONE. ALWAYS DIVE WITH A BUDDY!
This rule started when recreational diving was in its early days. It was necessary then to have help to put on your gear. From there, the crude equipment made it necessary to have another person there in case of emergency. These days we cannot imagine diving without a dive computer, BCD, or pressure gauge, but in the beginning, none of these devices were available as they had not yet been invented. Over the course of several decades, dive gear has become safer and easier to use. It has also become more self-sufficient and easier to manage alone. The invention and use of alternate air sources such as Spare Air, has even made it possible for a diver to handle an out of air emergency on their own. It is these advances that have allowed the idea of diving solo to gain momentum. Many dive agencies offer courses centered around solo diving. These courses are designed to increase the diver's independence and confidence in handling common diving situations which reduces their reliance on another person and the buddy system. However, despite these equipment advances and agency course changes, many people and places still agree that diving with a buddy is best.
If you decide you want to give solo diving a try, you must be willing to accept the responsibility for yourself should an emergency arise. You must also do your research. There are many places that do not allow solo diving and there are many parks that have rules that prohibit solo diving in park waterways. If you do find a location or guide that will allow solo diving, you may also have to show proof of your qualifications and experience with solo diving.
This just goes to show that there really are no RULES when it comes to diving, just guidelines and regulations that were created out of necessity or safety that have stuck with us through advances in technology and increased knowledge, for better or worse.