Why can't I sing high notes without straining?

First, a thanks to my Dad for inspiring this topic.  My Dad, a chiropractor in the Dallas area, is also is involved with the Garland Summer Musicals.  He isn't a professional singer/actor; he simply loves good theater and wanted to get involved.

This summer, he is singing in the barbershop quartet of the musical, The Music Man.  A few days ago, I got a call from Dad, asking for some advice on how to sing the high notes.

While there is some general advice I could give him, I had to tell him that as a voice teacher, I really need to hear someone sing in person to give tailored and specific advice to address any technical issue.  Just as one goes through several diagnostic tests at a doctor's office before the root of the problem is discovered, a good voice teacher has a series of vocal exercises to help discover the root of one's singing troubles.  Once the cause is uncovered, a voice teacher can design exercises and new ways of singing to address and eliminate the technical difficulty.

DWS