What do you call a person who always thinks that they're being used; a person who thinks that everyone else has an ulterior motive if someone asks them something?
For example, you might ask them if they want to go to the store with you, they'd immediately respond by saying "I'm not paying for anything!"
Or, if you ask them to hang out with you, they say "Do I look like an ATM to you? You're always trying to get me to spend money!"
This person doesn't give you a chance to finish your thoughts or intentions. They just assume that you want something from them.