realsocialskills answered:

Here’s how I’d explain it to people who are inclined to expect to have the use of other people’s computers:

Some people experience their computer/iPad/phone/etc as part of their body and find losing control over these things intensely distressing. Asking to borrow a computer can be like asking to borrow part of someone’s body.

Even for people who do not feel that way – Computers and things are expensive. Some people don’t like to share them, because they depend on them heavily and wouldn’t be able to afford to replace them.

Don’t put people in the position of having to tell you they don’t trust you not to break their computer. There’s no polite way to say that.

It can be ok to ask, but it’s important not to assume that the answer will be yes. And if you’re anticipating the need for a computer during the day, plan ahead rather than putting others on the spot.

For instance:

• If you know you’ll need to look things up during the day, and you also know that Bob always carries an iPad, don’t just assume that you’ll be able to use his.
• Either ask in advance, or bring your own
• If you’re going to need a computer for a presentation or to show a video or something, it’s very important not to assume you’ll be able to use someone else’s.
• Ask ahead of time, and take no for an answer if someone says no
• Putting people on the spot pressures them to say yes even if it’s not really ok with them
• Because it’s likely that everyone will think it’s their fault for ruining your presentation if they don’t agree to share their computer
• Don’t do this to people.

Some people are happy to occasionally allow friends and coworkers to use their computers. Other people aren’t. It’s ok to be unwilling to share, and the reasons why are no one else’s business. Don’t pressure people into doing things with their computer that they’re not really ok with.