Explain why, according to Guns, Germs, and Steel, humans and certain animals have unhappy "marriages."Guns Germs and Steel

In order to answer this question, you need to look at p.
168 and the following pages in the paperback edition of the book.  This is in Chapter 9,
roughly 6 pages from the end of the chapter.  There, Diamond gives you a number of
factors that can cause an animal to be undomesticable.  These are the factors that cause
the "marriage" to be unhappy.

The factors Diamond lists
are:

• The animal has to have a diet that can be
  compatible with being domesticated.


• It has to grow
  quickly enough to be worth keeping.


• It has to be willing
  to breed in captivity.


• It can't be too mean or hard to
  tame.


• It can't be too panicky because you need it to stay
  calm in enclosed areas and around people.


• It has to have
  a social structure that allows it to be
  domesticated.

If any one of these factors is
not present, the animal cannot really be domesticated.  This is why it is so hard to
find domesticable animals--they have to fit many criteria.