hello and welcome to HCM forum,
as you stated, you are experiencing pain in your lower left tooth, but have not mentioned whether the pain is intermittent or continuous, does it radiate or not, what are the aggravating or relieving factors.
therefore, i assume that the pain might be quite intolerable and that must be the reason why your dentist
suggested root canal treatment
RCT is performed when the nerves and vessels of your teeth get infected, in such a case, RCT is the only treatment or in severe cases extraction of the concerned tooth.
i would suggest you go for RCT quite soon before the pain becomes severe., delay in the treatment can cause spreading of infection to the sorrounding tissue(gums,jaw,etc) also.
kindly visit a dentist who is specialized in RCT's.
i wish you good health.