Why can I never say no.

OK so I have this friend and we hang out occasionally but not often. She will bring her kids over and they play and then leave. Anyway I can almost always count on her asking for something when she calls. So today the phone rings and I look at the caller ID and I see it is her. I immediately have to decide whether I should chance it and pick it up or ignore it. So I decide to pick it up. Mistake. First we start off with small talk and I tell her about how busy spring break has been and how I am tired of having all the neighborhood kids over etc etc. She tells me how it is going with moving and selling the house. After that she explains that tomorrow is the broker open house and she and her mom are taking her boys to the zoo but she has no place for her dog. He is a big loud dog and her place is quite small and it wouldn't help the place show well. So she asks if I can watch him for a couple of hours . I really want to say no because like I said this dog is loud and obnoxious and it is going to cause diruption to our home but nooooo I say "sure, no problem". Why Why Why!!!!!!!!!!! I never say no. I have babysat her kids, her dog (one time for 14 days) and she has never watched mine. OK so I have never asked her, but I always think that I don't want to impose on her. So there it is, I need to impose on her and I am gonna start. As soon as she moves and is settled in, I am gonna ask her to watch Emmy and see how it goes. If she says she can't then I won't do her any favors either.