Soap contains many chemicals such as foaming agents, colours, preservatives, artificial frangrances, caustic soda e.t.c.  Facial skin is very delicate and sensitive and soap shouldn't be used on it.
Your skin doctor might have told you to never ever apply soap on your face, and you have always wondered why is that. What could be so bad in soap that it cannot be used on the face.
Well, your dermatologist is right, using soap on face is one of the gravest skincare mistakes. Here is why,

Our skin is acidic, while soap is alkaline 
In simple words, the natural Barrier of your skin is made up of acid mantle. Anything below pH7 is acidic and anything above is alkaline. Generally, our skin's pH is between 4 and 6.5, even among those with a very oily skin. Soap, on the other hand, is the opposite - extremely alkaline. Soap messes with skin's pH and it's acid mantle, making skin condition worse. So, avoid soap for the best.

Soap makes your skin dry
Even people with an oily skin should not use soap on their face. Soap robs off natural oils, leading to a tighter, drier skin. If your skin is super greasy, use a face wash that is especially suited for that skin type. It will effectively wash off oils and dirt from skin's surface without disturbing the delicate pH balance. Washing your face with soap is like washing it with a detergent

Soap can severely damage your skin
When you wash your skin with the wrong kind of products, it looks dull, haggard and wrinkled. The list of wrong products includes soap. Soap is only one kind;there are no different varieties for different skin types. Horse chemicals present in soap may work to clean your body but they damage the delicate skin of your face. So, keep the soap away and invest in quality face wash instead.
If you wish to use the ultra-moisturising varieties of soap marketed by various brands these days, consult your skin doctor first. Make sure you use a product that keeps your skin young and beautiful.