St. Catherine of Siena said that human beings can't live without love, that they are sustained by love because love is the substance of their being, the "stuff" from which they were made. St. Augustine, too, in Confessions II, II.2 says that the wild days of his youth were really a search for reciprocated love.
Indeed, I don't know if there is a greater happiness than the one brought about by the realization that an other person, someone who did not grow up with you and is not related to you, loves you and expresses her desire to spend her entire life around you. I judge that the happiness of this realization consists in the fact that such a love surpasses all your merits and efforts, that you could never have created it whatever you might have done. No qualities, no talents, no pleasant features — these all being uncertain and transient — could deserve the unconditional commitment of the other. In other words, a love of this kind makes us happy because it's given to us as a gift.
This gratuitous aspect of love endows it with a certain magnificence and makes it to be the essence of a happy life. Even familial love gains a different value when it survives mistakes and grave trials. In a way, it is to be expected that your parents and family members to love you, so long as you haven't done anything wrong; this sort of love is instinctual. But it presents itself as possessing a quality apart when this sort of love persists in the aftermath of a grave error such as to erase any merit, to cancel any expectation of forgiveness.
The gift makes happy, then, from which it clearly results that human beatitude cannot consist in our own accomplishments. We cannot effect it through our own efforts. Rather, in consists in grace, in the love which the other gives freely. At the very most, we can — indeed, we ought to — prepare ourselves for the gift of happiness, making ourselves persons who are "worthy" of receiving it. Naturally, we cannot force another person to love us, but we can do what we can to be lovable, to be the sort of persons who call for a reaction of love from the other.