Tying together relationship between the two strongest emotions, pain and love. Religiously, christ suffering and being crucified symbolizes the pain he went through because of his love for all manking and dying to free mankind of their sins. Also, shown with parents, children. A mother is willing to endure a significant amount pain, even death because of her love towards her newborn. "Pain is love"..
many other examples of this..
many many examples..suffering and enduring pain because of a greater love...
"pain is love" - used as an expression