I love Zero so much! He's the best character in the history of anime! :D
Apart from "because he's so attractive and lovable" stuff, it's because:
It's not that a character has to be perfect to be loved. Because Zero is not perfect. Well, I love him for my own reasons... also because he really is just a strong character. He's well done, people can often sympathize with him even when he's being ridiculous.
Unlike Kaname, Zero is caring and understanding. He doesn't try to take Yuuki away from Kaname like Kaname does from Zero. No, he let's her go. He picks more pain and sorrow in his life just to give Yuuki happiness and less confusion and regrets.
His sad and painful past... When I saw how much pain he's through, I felt so sad for him. All this time I wanted to be close to him, to comfort him even though he's just an anime character. And all this saddens grew to affection. I really got attracted to Zero.
The way he's always by Yuuki's side. He risks his very life just to keep Yuuki safe. He cares for her from the bottom of his heart! He would do anything for her. Just because he's so closed and cold does not mean he doesn't have feelings! In fact, he has even more feelings than Kaname! I can't believe there are people who call Zero unfeeling... They might have been too busy drooling over Kaname to notice Zero's feelings.
His personality. I don't know why you hate his personality, but it's really not as you say. Should he be fun when he got his family killed years ago? Should he be happy when he's so lonely? Just because a person doesn't show his feelings, does not mean he doesn't have them!
When was he going around being jerk to everyone? If he was like Kaname, he would never get the students away from the path that the Night Class walks! That's really sweet of him. Yuuki couldn't handle the job herself! He's very helpful.
He's cold and closed because of his past and loneliness. Imagine you being in his place. How would you be? I really doubt you'd feel sunshine and rainbows.
Zero can't control the "him" that thirsts for Yuuki's blood. He would never do it to her without her permission. She just sticks her neck out to him and asks him to do it! He hesitates. He doesn't want to. But his vampirism is just the same as being possessed by a demon or something like that. He can't control it! That's why he tried to stay far from Yuuki. But she asked it herself. She doesn't want him to leave. She needed him! And he needed her!
So I guess that's enough. I actually have plenty more reasons but that's enough. In my opinion, I say Zero isn't hatable at all. It's just you Kaname fan girls who only judge a guy by his appearance. Don't hate me for telling the truth please. You should read my reasons why I hate Kaname. That way you'll know it's the truth. Thank you.