Metta is cultivated deliberately, usually beginning with phrases addressed first to oneself — may I be safe, may I be happy, may I be healthy, may I live with ease — then widening the circle to loved ones, strangers, difficult people, and finally all beings.
The practice is not contingent on feeling. One offers the phrases; the felt sense develops over time, by repetition. Metta is one of the four brahmaviharas (“divine abodes”), with Karuna (compassion), mudita (sympathetic joy), and upekkha (equanimity).