While Serana is one of the best followers from the Dawngaurd expansion. Some of the conversations about her never getting attention from her dad, Harkon and how she was left to herself as a newly turned vampire to wander the castle and how she uses her lonliness to follow you around like a lost puppy dog the entire time. She also is obviously rebelling against her father because she knows deep down inside that something is going to happen to her. So she rebels whether you're a vampire or in the Dawngaurd.
The whole Dawnguard storyline seems to be around Serana having issues with her dad. Whether you journey into the Soul Cairyn, and meet her mother. Well, the conversation goes back to Harkon, yet again. Even when you first get to the castle.....we see that Harkon isn't at all surprised to see his long lost daughter. He's mainly concerned about the elder scroll that she has. But why be concerned about the elder scroll if Serana is the key to the Tyranny of Sun?
However its very comical how Bethesda decided to give Serana major issues. Its obvious that Bethesda gave Serana abandonement issues as to motivate her into helping you on either side.
Could it really be that Serana see's the Dragon Born as a father figure since her own father had little to nothing to do with her?