Why does teeth bleaching ruin tooth enamel?

It does not: I have successfully bleached hundreds of teeth with no damage to the enamel. The only side effect is occassional gum irritation or tooth sensitivity.

It does not: There have been many studies done on the safety of bleach on enamel and not one has proved it is permanently harmful. (yet?).

For prolonged period: Of bleaching can weaken the enamel. Carbamide peroxide at 16% or below is acceptable. Avoid very low ph of some bleaching agents because they can demineralize the teeth. Bleaching or teeth whitening monitored by dentist is safer so he can also check the condition not only of the teeth but the soft tissue as well.