This is a question I've asked myself on any number of occasions (and despite my own thoughts below, I'd like to hear your thoughts on this as well). For some reason, most vampire literature with a vampire protagonist has that protagonist hating him/herself and his/her condition (switching to 'his' after this sentence, for the sake of brevity).
Let's be honest with ourselves... if we could gain immortality, immense strength, and the ability to control people's minds in exchange for having to suck human blood and stay out of the sun, the greater majority of us would probably leap at the chance. Humans are selfish creatures, and the advantages seem to far-outweigh the disadvantages on the face of it.
One common answer to this is morality. To be honest, I think this is the second worst answer of them all. Yes, in the case of a vampire setting where the vampire has to kill the subject or infects anyone he bites, it makes sense for there to be a moral issue. However, if that is not the case, this one doesn't pan out. Sure, drinking blood sounds evil in and of itself... but if you aren't human, it isn't cannibalism, now is it? Hedonism? Is there anyone in a first-world nation that isn't at least a little hedonistic?
Another one is a sense of isolation. Now this one makes a bit more sense as a negative for vampirism... but not for sucking blood. Sure, it might be hard to make friends with your food, but it wouldn't be the first time. No, the issue here is lies. By nature, vampires need to hide themselves, since they are well... scary. I mean, if something essentially eats a part of you to survive and looks similar to you, how can that not be scary in a visceral sense? So yes, the isolation is a good reason to be afraid of your own vampirism if you are a vampire.
A sense of normalcy. This is the one you see the most in VNs, and I honestly think that it is an abomination. It is the worst answer. Almost every vampire protagonist in a VN wants to regain their 'normal life', and this often results in them taking their anger out on the people who saved them and/or love them. Sure, you pay a price for your vampirism... if you can't go out in the sun (a rarity in VNs), then it is hard to go to school. If you have to suck human blood, then you can't really be called normal. However, that sense that normalcy trumps everything (no I don't mean the president) is ridiculous. I honestly find this kind of attitude annoying as hell in a protagonist, and it is only the ones who don't linger on it constantly that I'm willing to forgive.
Violent instincts... now this one is laughable. 'Vampires have violent instincts!!!' Umm... hello? What race goes around killing people for stupid reasons like religious affiliation, what side of a line on a map they live, and who has a better cow in their barn? *snorts contemptuously*
In the end, vampirism in fiction is a trade-off... and self-hating vampires who stay that way without a good reason always strike me as fake (Toshirou from Vermilion has good reason, but most don't).