Thomas Aquinas

“ On the other hand sorrow arises from love, either through the absence of the thing loved, or because the loved object to which we wish well, is deprived of its good or afflicted with some evil. Now charity is love of God, Whose good is unchangeable, since He is His goodness, and from the very fact that He is loved, He is in those who love Him by His most excellent effect, according to 1 Jn. 4:16: "He that abideth in charity, abideth in God, and God in him." Therefore spiritual joy, which is about God, is caused by charity. ”

“ The mourning that merits happiness, is about those things that are contrary to happiness. Wherefore it amounts to the same that charity causes this mourning, and this spiritual joy about God, since to rejoice in a certain good amounts to the same as to grieve for things that are contrary to it. ”

“ When each one attains to happiness he will reach the term appointed to him by Divine predestination, and nothing further will remain to which he may tend, although by reaching that term, some will approach nearer to God than others. Hence each one's joy will be full with regard to himself, because his desire will be fully set at rest; yet one's joy will be greater than another's, on account of a fuller participation of the Divine happiness. ”