Bleaching of vital teeth is contraindicated in sensitive teeth because bleaching agents like hydrogen peroxide can exacerbate tooth sensitivity by penetrating enamel and dentin, irritating the pulp. This can lead to discomfort, pain, and potential pulp inflammation. In contrast, discolored teeth, mild fluorosis, and tetracycline staining are common indications for bleaching. Managing sensitivity before or instead of bleaching avoids patient discomfort and adverse outcomes. Therefore, sensitivity is a critical factor that dictates the appropriateness of vital bleaching.

Reference: Principles and Practice of Operative Dentistry, Satinder Dhawan, 3rd Edition.