First off, I'm pc so it's not like i've been able to nitpick and interact with her and stuff but here's why I DESPISE Serana. So, multiple times during the dlc you can trade dialogue with Serana and others i.e. her mom that seems to imply that the player and Serana are the most loveful lovers that ever loved their loves. But there's one small problem; you know how she's a vampire, you know, those things made by a dood called the "King of [censored]"? Whom the whole Volkihar family had to do nasty not-so-fun-even-if-you-consent type stuff? What type of person does that kind of vaguely hinted at stuff to become a daughter of coldharbour and turns around and says (probably paraphrased) "It's ok, I understand if you don't want to be a vampire to get into the soul cairn, I don't want to make you do anything you don't want to do"? My character "Wait, uh, you're a vampire, your a thing that kills people for snacks, your dining room in the castle has pieces of human remains scattered about. Why do you care what I do or do not want to do?"
I get that you can't make a game with too much graphic stuff, but to not even make Serana a mean person. It makes me even more mad when the player/Serana can't even get married since they're obviously the best of buds. Surely there has to be some kind of vampire chapel that they can go to, right? /rant.
I'm not mad at Beth for doing this wierdness of character that is Serana, but I just wish they made their games a bit more visceral.

She likes you because you are the first person she tasted blood from in about 2000-4000 years. This is just me guessing. Now take this with a grain of salt since this is a note from UESP wiki.

If you side with the vampires, Serana will be more violent as opposed to with the Dawnguard which may cause a problem if you aren't kill-happy.

I haven't played the DLC myself only saw the main quest line for each side on youtube.

The fact you released her? I don't know. The only NPC's you really see her interact with are her parents. She seems a lot nicer to you than them.

I think you have to offer up about 1000 souls before Molag Bal even gets a glint in his eye for you. So I would say that doesn't make you a nice person to start with.

I don't see this romantic lover talk you're referring to, I just see a person making sure a friend is okay.

I can understand it coming from her if you were on the Vampire side (even though it's not applicable your already a Vamp), the whole lovey dovey thing, I just get the impression that either Beth. created her for the Vampire side and either liked her so much they placed her into the Dawnguard side as well, or they just got lazy and there was going to be another endearing NPC for the Dawnguard side.

Could explain why people are seeing these kinds of inconsistencies.

If she wasnt, people would say "Why does beth make all vamps mean and evil? They dont have imagination!" Or some such crap. She's fine the way she is, except the fact that we cant marry her. I dont see anything wrong with her personality. Maybe its your dovahswag she likes so much, lol.

Because she is a teenager and not evil.

She makes more sence on dawnguard side. Coming to ask for help since the mission her father gave you sounds fishy. Not to mention that on Dawnguard side she explains a lot more and faster about here being locked away by her mother.

Oh I see, there was supposed to be another endearing NPC who was with the Dawnguard side and decided that helping Harkon and the Vampires was their true calling in life. Now that makes so much more sense.

You don't seem to understand anything about Seranas' character. She doesn't go to the Dawnguard side because she loves the Dawnguard she joins so she can stop her father who intends to kill her and blot out the sun. It's a mutually beneficial truce.

Because the angsty teen demographic needs a character it can relate to.

Well will agree to disagree.

I don't. You option makes no sence bacause it's already has been done. You get an option to betray dawnguard. Atleast with Serana you get diferent starts and reasons in the begining.

I'm of the impression that you don't like anything anybody does. Bethesda could have made the perfect game and you would still have something to complain about.

As stated earlier, your blood is the first blood she has had in 4000+ years so she instantly becomes infatuated with you and wants to turn you into a Vampire as her way of repaying you since to her its the greatest gift she and her father can give. (She knows full well what her father was gonna say to you when you first go to the Castle for saving her, if denied she later on forever gives you the option to become a vampire..even if you cure it and want it back several times over...)

So its like having six. You both get an orgism from the bite. And you can do it multiple times.

I...guess you can think of it that way. lol

More than your spelling, I suppose.

It is "sense" nor "sence" . Just a little grammar correction for you.

Anyways, not all vampires are mindless killers who want to eat you. Serana does kill people, she is more violent against the Dawnguard, and she even sarcastically told Isran she would be remembering him next time she has a "snack". Obviously, she might be friendly to the player, but she's still a killer. A "teenage" killer.

Janus Hassildor is an example of another "nice" vampire. He treats the player with civility, but he still acknowledges a female vampire hiding in his castle and uses her to kill off criminal scum and collect blood to feed his wife, Rona. He also said he "embraced the changes in himself" meaning he loves being a vampire.

since when was hundreds of years old counted as a "teenager" ?

She has been around the block..just ask Molag Bal.

She isn't a teenager, shes a woman. =P

Hence the quoatations around the word "teenager".

She's still a young person in body and in mind. Look at Babette, she may be a little girl in body, but her mind is obviously mature throughout the centuries. Serana? She was imprisoned when she was young. She might be thousands of years old, but she never had the chance to fully mature and develop, as obvious by her attitude and ranting about her parents and her being locked up.

It is the same as a young girl who was put to sleep for centuries upon centuries upon centuries. It would feel like a day or two passed in her slepe, but when she wakes up her mind would still function the same way.

She's a teen mom. Her daughter has been grounded by Daddy for love afairs

These are just my thoughts on it:

Maybe she is nice to you because you tend to be nice to her. Even if you chose the less nice dialog options you still help her and you still make yourself trustworthy in her eyes.
Another thing is that she explains that she was expected to participate in the ritual that turned her to a vampire since her family was followers of him and it was tradition to offer women to him on his summoning day. She doesn't express anything that indicate that she had an actual choice in the matter or that she sacrificed anyone for the priviledge.
She seems to be one who just got caught up in it all as a result of her parents.
And lastly, as a Dawnguard you save her life by vouching for her to Isran.

I think she have plenty of reason to like the player to be honest. That she is a vampire doesn't mean she doesn't have emotions etc.

Actualy she expresses quite a bit of hate for the ritual. And as you sayed even her mother admits that she had no choice.