Self-respect at Core of Ethics

Plato defines ethics as behaviors that do not harm others - or ourselves.  Aristotle was the student of Plato, and he, as you observes, footnotes his teacher's advice about treating ourselves well, not just others, as central to ethics.  As you observe, if we are happy with ourselves it "spills over" into our other relationships.  The same "spillover" occurs if we are unhappy with ourselves.  Being physically, mentally, spiritually healthy "spills over" into our personal self-acceptance and acceptance of others in positive ways.  Liking ourselves does not mean going on ego trips.  It means respecting ourselves as person's of worth; therefore, we need to treat ourselves with proper value, e.g. physical fitness.  Yes, it starts with us as individuals.