I really need some very good reasons why fast food is bad for us. please elaborate answers.

Toxic: Any time oils are heated they start to break down. The more saturated it is(solid at room temp) the more resistant it is to heat damage. Fast food restaurants typically use liquid oils. As those are used in fryers they become more toxic. They also use hydrogenated oils in many products and those are toxic. Last - high levels of sugar and starches are bad (most of fast food).