She got over him, so it's only fair he should be allowed to be over her without a big deal being made about it. His soul's permanent now, and she didn't expect him to spend the rest of forever in love with her.
She's the slayer, and it never would have worked out for so many reasons. She's got Scott, and he can make her laugh, not to mention see her in the sun, so she shouldn't be hurt that Angel's moved on. He has a right to be happy, and if... if that's what makes him happy, then good for him.
It's just that... Okay, it's a big 'yay, Angel, you can be in love now' and everything, but did it have to be with one of her friends? Do they have to hold hands under the table when they're researching and do the couple-y things around her that she never could have done with him?
And since when does Xander like men anyways?