is most likely to be recommending a root canal
because you have evidence of nerve root exposure or infection. A cavity occurs when bacteria erode through the outer surface of your tooth (the enamel) creating a "pit" which can serve as a location of infection. When this pit becomes deep enough to involve the nerve at the center of the tooth there is a risk of infection of the nerve (also called the "pulp") called pulpitis. This is usually very painful, and is one of the most common reasons for emergency visits to the dentist.
When cavities are shallow, they can be drilled out and covered with a "filling
." In order to treat a cavity that has become deep enough to involve the nerve at the center of the tooth, a "root canal" is necessary, whereby a canal is created in the side of the tooth to allow any buildup of infectious fluid to drain, and the nerve pulp is removed to relieve the acute symptoms of pain. Failure to treat an infected tooth can lead to eruption of the infection through the tooth (destroying the tooth), or spread of the infection to local (forming a nearby abscess) or deep (spreading to the neck or beyond) tissues.
It is always within your rights to seek a second opinion, but if you have been told you need a root canal, you need to seek your second opinion urgently, as infection of a nerve root can be very painful and potentially complicated. I suggest you make an appointment to see another dentist as soon as possible, to see if a root canal is truly indicated.