A moment of weakness during Thorin's captivity has the elven lord with child. Thranduil shows up on Erebor's doorstep, having fled his own kingdom; but Thorin has not forgiven his old paramour and although allowing him housing for the duration of his pregnancy, is adamant in forgetting their love and marrying Bilbo. Thorin/ Thranduil Thorin/ Bilbo Baggins

Thranduil is a model who is frustrated with how people treat him. Thorin is a luckless (and currently homeless) painter who is looking for his Arkenstone, the thing, place, person or state of mind which will allow him to paint masterpieces. Maybe those two souls are meant to meet.