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.