“As the French say, aimer est souffrir, to love is to suffer, and therefore, to say that love is essential to the Being of God is to say that in one way or another suffering is essential to his nature. That is to say something our forefathers would have declined to accept. They would have retorted that God, as Father, must be above or beyond suffering. If, however, we take seriously the declaration "God is love," how can we avoid the entailment that suffering is essential to the divine Being?”

The Magnificent Defeat (1966)

Love and Death (1975)

Need-love says of a woman "I cannot live without her"; Gift-love longs to give her happiness, comfort, protection — if possible, wealth; Appreciative love gazes and holds its breath and is silent, rejoices that such a wonder should exist even if not for him, will not be wholly dejected by losing her, would rather have it so than never to have seen her at all.
The Four Loves (1960)

"Oration VII": "To the Cynic Heracleios", as quoted in The Works of the Emperor Julian (1923) by Wilmer Cave France Wright, p. 105; also in Hidden Wisdom: Esoteric Traditions and the Roots of Christian Mysticism (2005) by Gedaliahu A. G. Stroumsa, p. 25
General sources

The Tragic Sense of Life (1913), VIII : From God to God
Context: Not by way of reason, but only by way of love and suffering, do we come to the living God, the human God. Reason rather separates us from Him. We cannot first know Him in order that afterward we may love Him; we must begin by loving Him, longing for Him, hungering after Him, before knowing Him. The knowledge of God proceeds from the love of God, and this love has little or nothing of the rational in it. For God is indefinable. To seek to define Him is to seek to confine Him within the limits of our mind — that is to say, to kill Him. In so far as we attempt to define Him, there rises up before us — Nothingness.

On her dark night of spiritual desolation amidst devotion, in a letter addressed to Jesus, as quoted in Mother Teresa: Come Be My Light (2007) edited by Father Brian Kolodiejchuk, p. 192; regarding this quote, Fr. Kolodiejchuk writes: "...when addressing Jesus — that is, in prayer — she could express herself with ease. Fufilling her confessor's request, she sent to him a letter addressed to Jesus, enclosing it with her letter dated September 3, 1959." https://books.google.com/books?id=P4cqT0nK_joC&pg=PA192&dq=%22when+addressing+Jesus+-+that+is,+in+prayer+-+she+could+express+herself+%22&hl=en&sa=X&ved=0ahUKEwjk0IOm5vTOAhVF1x4KHYdRDE4Q6AEIHDAA#v=onepage&q=%22when%20addressing%20Jesus%20-%20that%20is%2C%20in%20prayer%20-%20she%20could%20express%20herself%20%22&f=false
1950s
Context: My own Jesus,
They say people in hell suffer eternal pain because of the loss of God – they would go through all that suffering if they had just a little hope of possessing God. In my soul I feel just that terrible pain of loss, of God not wanting me, of God not being God, of God not really existing (Jesus, please forgive my blasphemies, I have been told to write everything). That darkness that surrounds me on all sides. I can’t lift my soul to God – no light or inspiration enters my soul. I speak of love for souls, of tender love for God, words pass through my words sic, lips], and I long with a deep longing to believe in them! What do I labour for? If there be no God—there can be no soul.—If there is no soul then Jesus—You also are not true... Jesus don't let my soul be deceived—nor let me deceive anyone. In the call You said that I would have to suffer much.—Ten years—my Jesus, You have done to me according to Your will—and Jesus hear my prayer—if this pleases You—if my pain and suffering—my darkness and separation gives You a drop of consolation—my own Jesus, do with me as You wish—as long as You wish, without a single glance at my feelings and pain... I beg of You only one thing—please do not take the trouble to return soon.—I am ready to wait for You for all eternity.

Source: Peace Is Every Step: The Path of Mindfulness in Everyday Life

Bk. XIV, ch. 15
War and Peace (1865–1867; 1869)