The most common cause of dental implant failure is peri-implantitis, a condition where the tissues surrounding the implant become inflamed due to bacterial infection. This inflammation can lead to the deterioration of the bone supporting the implant, ultimately causing it to become loose and fail. Other factors contributing to implant failure include poor oral hygiene, smoking, and underlying health conditions like diabetes that affect healing. Early detection and proper maintenance are crucial to prevent peri-implantitis and ensure the long-term success of dental implants.