Source: Wild rumors (may also be false).

Germ phobia

Donald Trump has been very open several times when he has said that he does not like to shake hands with other people. He also refuses to touch the ground floor button of a lift.

It does not stop there. He has also figured out that teachers have more germs on their desks than other people, so especially shaking hands with teachers is a no.

Trump: “I’m going to do everything in my power not to shake hands with teachers. They have 17,000 germs per square inch on their desk. That’s ten times the germ rate of those in other professions.
It’s absolutely swarming with germs. In fact, from now on I’m just going to walk up to my office. It’s pretty high up there, but I could use the exercise.”