One of the main reason this pairing works for me and I assume for a lot of people is that fact that these 2 characters can relate to each other very well. More than any of the other Titans at least. Both of them are very closed off loners/outcasts with both of them coming from a troubled pasts. With Damian being the grandson of the demons head Ra's al Ghul where from birth he was told it was his destiny to lead the league of assassins and with Raven's father being a literal demon Trigon. Both characters have dark destinies that they try to overcome and thanks to each other they are able to do so.