@PaulieFlyn10,
Sometimes it's due to not qualifying her earlier. If you just started talking to her, haven't told her anything really unique about her that you like, then asked her out, the first thing on her mind is going to be, "Wait. He doesn't know anything about me. Why does he want to meet? Does he just ask out any girl who'll talk to him?"
If you think it's due to lack of qualification (and if you really didn't tell her much you like about her, it's a safe bet it's probably that), then you just spool off a few quick cold reads or unique observations about her that make her feel like YES, this is actually personal, and YES, you are seeing her as special, not just some random chick. e.g.:
"Well, you know, I really like that you like animals -- that tells me some good things about you right up front. You're health-conscious with your pilates so I know we vibe well on that front. And our conversations so far have been fun. You liking [RANDOM BAND YOU BOTH LIKE] definitely gets you some bonus points. I think it'd be nice to see if we vibe just as well in person."
You will also get girls doing this because they have self-esteem issues, or are inexperienced, where even if you've qualified them a lot they still feel awkward and just give an instinctive pushback for you to get around. If you're in-person (I know you met this one online) it's easy to tell because she looks a bit nervous/hesitant and it's more of an inquisitive, unconfident, "Why?"
For that you just tease, then qualify. Like so:
YOU: We should hang out sometime.
HER: You mean you want to go out with me? Why?
YOU: Well, the truth is I'm a human trafficker and we need more slaves. No, I'm just kidding. Well, duh! I've had fun talking to you and I think I could potentially like you. I'd like to talk with you more again and find out for sure. Is that a good enough reason?
Then you get the ones where it's just a standard test. You didn't do anything overly bad; she just wants to see how you react when she throws out a curveball.
If you're in-person, you know this because you qualified the girl well and the two of you had a good vibe, and when she asks why it's in a confident, smiling way with her looking at you expectantly, clearly ready to gauge your response.
Some absurdist future projection is my preference for these if she has a sense of humor:
YOU: We should hang out sometime.
HER: [confident smile] Why?
YOU: Well, you know, as much as I love stealing puppies from the dog park, it's really a two-person operation. I think with your build and physique you'd be just right for it.
Or if she hasn't shown as much of a sense of humor:
YOU: We should hang out sometime.
HER: [confident smile] Why?
YOU: Oh don't give me that. We've had a nice conversation and we get along. It'd be weird if we DIDN'T meet. Here, hand me your phone, I'll put my contact in for you.
Chase