I want to ask my girlfriend for a reason since she tells me no all the time.
Why do you need a reason why your girlfriend says no
I am just tired of taking no for an answer every single time.
You are putting a lot of undue pressure on your girlfriend by asking her so many questions. You need to accept that her answer is no and then leave the situation along because you are nagging her.
I'm only having a fun time, what's wrong with that?
If you are having fun by repeatedly asking her questions then the issue may be you and not her. If she tells you no when you ask her something, there is not point in asking her again right after. Just understand that her being happy will make you happy as well, so playing jokes on her is not nice.