Maybe she just doesn't want to inconvienence you, and/or feel less of herself for relying on the man next door. Some, alot of women, single women especially are like that, it doesn't mean she doesn't appreciate it...she just doesn't want to have to rely on the nearest person, and maybe she thinks you have better things to do then fiddle on her car (not saying there's anything wrong with that, as I'm a member of this board too), but maybe she doesn't understand that it's a hobby for you.
Don't jump the gun and say that all women don't care about their vehicles, I know a lot of women who pay more attention to leaks, car not running right, etc then most of their spouses...it's just that not all of them have the time or the know-how to work on them, so they take them to who they know.