There are several factors that can weaken a strong marriage. Lack of communication, unresolved conflicts, loss of intimacy, financial problems, infidelity, and a lack of trust can all contribute to marital difficulties. Additionally, external stressors such as work pressures, family issues, and health problems can strain a marriage. It is important for couples to address these issues early on, seek professional help if needed, and actively work towards strengthening their relationship through open communication, trust-building, and shared efforts.