Since they first met, Robin was teetering between whether or not he should trust her, but upon seeing her involved in acts such as reuniting Cyborg with his estranged father he began to trust her bit by bit. He was one of the few people that still believed her over Trigon's coming after Zatanna spilled the beans on her using her powers on Kid Flash.
After Raven was first free of Trigon's influence. She began to experience emotions she never had before, such as love. She fell in love with Robin, who at the time became Nightwing. However when she assumed the feeling was mutual, she inadvertantly used her powers to manipulate his emotions making it seem his love back to him was genuine. This in turn caused them to experience her first on panel kiss as pictured above. Although Starfire straightened her out on the differences of love, it is hinted that though Nightwing loves her platonically, but she loves him still romantically, but never tries to get between him and Starfire.