What is Love? -- 'Abdu'l-Baha explains

Know thou of a certainty that Love is the secret of God’s holy Dispensation, the manifestation of the All-Merciful, the fountain of spiritual outpourings. Love is heaven’s kindly light, the Holy Spirit’s eternal breath that vivifieth the human soul. Love is the cause of God’s revelation unto man, the vital bond inherent, in accordance with the divine creation, in the realities of things. Love is the one means that ensureth true felicity both in this world and the next. Love is the light that guideth in darkness, the living link that uniteth God with man, that assureth the progress of every illumined soul. Love is the most great law that ruleth this mighty and heavenly cycle, the unique power that bindeth together the divers elements of this material world, the supreme magnetic force that directeth the movements of the spheres in the celestial realms. Love revealeth with unfailing and limitless power the mysteries latent in the universe. Love is the spirit of life unto the adorned body of mankind, the establisher of true civilization in this mortal world, and the shedder of imperishable glory upon every high-aiming race and nation. (‘Abdu’l-Baha, ‘Selections from the Writings of ‘Abdu’l-Baha’)