The answer to the crossword for the term 'Cause of 'I' Strain?' is EGOISM, which is defined as excessive or exaggerated self-importance or self-interest. It is a concept in psychology that refers to an individual's tendency to prioritize their own needs and desires over those of others. EGOISM can lead to feelings of isolation, resentment, and frustration, and can be a major source of conflict in relationships.