How to answer "why" with no good answer?

I need help. So I request something and they as me 'why'. I really don't have a good reason, and I give some reason, and they ask 'why' again. I already struggled to come up with an answer the first time, how do I answer 'why' with no good answer?

ex: I asked a girl for her number, and she asked 'why?' So I said that I wanted to talk to her. and she asked me 'why?' again, and I said 'to get to know [her] better." and she asked 'why?' again! WTF??!!! I wanted to slap her. -__-

ex2: I am working in pairs on a project that doesn't matter who works with whom. Cute girl I don't know in a different group, go talk to her, ask if you can be partners. She agrees, so I ask her current partner if I can be her partner instead of him/her, who then asks me 'why?' I have no answer for this 'why', because I like her?? because she's pretty? because I want to get to know her? all these answers kinda suck bad for reasons to switch partners- especially since it doesn't matter who you work with, and risk being way too obvious. (esp since I don't know the girl either :/)

Need help guys/girls! Thanks!