I did a dental filling on my teeth & now it need a root canal. WHY?

You should go check with another dentist as a second opinion. I don't trust this dentist of yours. Any changes made to your tooth could be fatal and irreversible. Once your root's gone, it's gone.

Sometimes this happens when the filled area is rather large and deep, so there is only a thin section of dental bone separating the bottom of the cavity and the root of the tooth. The root becomes irritated from the drilling and from the usage of the tooth while chewing.

It is unpredictable how your root will react when the case is that you need a large filling, and it is hard even for an experienced dentist to predict the outcome. The best outcome is that the tooth will survive without root canal and the filling is enough. This is why the dentist will always try to make a filling first if the root of the tooth isn't showing signs of infection or irritation.

When a root is infected it will show up in the x-ray, the diagnosis is completed if the tooth has symptoms such as you described; pain, sensitivity to hot&cold or pressure. In your case the root wasn't acting up before, and was probably not yet so irritated that the inflammation would show up in the x-ray.

Large fillings can do this to other teeth as well, it can take anywhere from 1 week to several years for the root to become infected. It could also be because of improper hygiene when doing the filling, if bacteria from your saliva got into the bottom of your drilled cavity, it could have lead to the root getting infected.

Get a second opinion on the tooth, show the x-rays and ask if there is a chance that the other dentist made a mistake. You could then get some compensation.