A friend always has to be diplomatic, to avoid meeting people who might interpret a friendly encounter as something else. There are moments when wants only to set the boundaries of the relationship so that there is no interference in your private affairs. Otherwise you might feel at a loss especially when you find that the party who invited you is invasive of your personal space. The same can be said in living situations when there is one computer in the home and it stores private information.
Telling people that they cannot use your computer while you are away is understandable especially if they cannot be trusted or have shown to be unstable in character. I would not want to be used for certain conveniences in a friendship either and these days with the need of people to contact others all day long through the internet or by cell phone, it is easy to be taken advantage of especially if you show kindness towards a particular individual. They may take it as a passport to always asking.
Similarly if I were going to be invited to an expensive restaurant, I would tell the person that I couldn't afford it and would not want that person to pay on my account. If that person insisted on paying I might even add that if he did, I would not want to continue going out to eat because I was uncomfortable with someone looking after me. It is another way of saying that I don't appreciate it when people force certain issues without considering another person's comfort zone.
