Thomistic ethics best answers this question on Man’s Final End and how we arrive at this conclusion from a Catholic perspective. Thomistic ethics begins with the end in mind; and defines happiness as knowing, loving and serving God. We know this as this corresponds with our natural faculties/ Reason which enables us to think about, know, and love God.
Man’s Final End and the path to perfect happiness, however, goes above and beyond that. True happiness is receiving the beatific vision of God and spending eternity with Him. The beatific vision is a supernatural gift from God where we are blessed to see God face to face. The beatific vision is when the soul is granted perfect knowledge of a perfect God so as to contemplate God and be united with Him. A soul in this state of beatitude understands God as a Being who is all good, all beautiful, all amiable, and the soul enjoys all perfection.
We know our final end is God Himself as He is the first cause who is all-perfect and lacks nothing. God did not create us to increase His glory, add to His happiness, or intrinsic value since He is the Infinite Good. He created us to manifest His glory and perfection to us. Our Final End is thus to manifest God’s glory; which unveils His perfect power, wisdom, and love.
We are able to achieve our Final End because of God’s agape love, mercy, and charity. He promised us a Savior after the sin and Fall of Adam and Eve. This Savior was Jesus who came from Heaven to earth, to die for our sins, so that God may raise us back to Heaven. God grants us this supernatural gift of beatific vision through divine revelation and the supernatural virtue of hope. He has and continues to call us towards our Final End, which is to experience Love in its truest and purest form – God Himself.
* Image was taken at my Conversion Experience Retreat (in July 2016) at the Singapore Catholic Spirituality Center.
 Coppens, Charles. A Brief Text-book of Moral Philosophy. Schwartz, Kirwin & Fauss, 1895.
 Mortimer, Robert Cecil. “The elements of moral theology.” (1947).