When we love a thing by desiring it we apprehend it as demanded by our well being; when we love another in friendship we wish good to him just as we wish it to ourselves, we apprehend our friend as our other self. St. Augustine remarks that well did a man say to his friend, "Thou half of my soul."
Love is like the efficient cause of real union, because it moves a person to seek and find the company of the beloved, who fits in as part of one's life. Love is the formal cause of the union of affection, because love itself is this bond.
-St. Thomas Aquinas

Friendship should be true, right and holy.
When we love our fellow only in so far as they are of use to us, they are not truly loved; we are really loving ourselves and using other people as though they were for our convenience. We do not truly love them as ourselves. He who loves another merely for his own profit or pleasure stands convicted of selfishness. Charity seeketh not her own, but the good of the beloved. Then secondly, the love of another should be fair and tight, which means that a greater benefit must be prefered to a less. The good of soul must hold the first place, then afterwards comes the body, and lastly external possessions.
-St. Thomas Aquinas

Four general reasons can be brought forward to show that perfect happiness consists neither in riches, nor in honor or fame, nor in power. Of which the first is that perfect happiness is not compatible with any evil. The second is that happiness is self- sufficient; once obtained no other prize is wanting, such as good health and wisdom. The third is that no harm results from happiness, whereas sometimes riches are kept to the hurt of the owner, and this may also be the case with the other goods we have mentioned come from external causes and often from good luck.
-St. Thomas Aquinas