Microleakage is the most frequent cause of failure in composite resin restorations because it leads to the infiltration of bacteria and oral fluids at the tooth-restoration interface. This compromises the seal, causing sensitivity, staining, and eventual secondary caries. Additionally, microleakage results from polymerization shrinkage, which creates gaps and stresses at adhesive bonds. Proper adhesive techniques and incremental layering help minimize microleakage but cannot eliminate it completely, making it the primary clinical challenge in composite longevity.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.