Finding happiness is not as simple as Abel suggests, and, in fact, Abel can hold to this view only because he spends most of the book in a cocoon oblivious to his wife's pain, his son's challenges and the existential threat to his people posed by the Nazis. Abel uses the self-fulfillment of his work and the security of his faith to insulate himself from the unpleasant realities around him, but ultimately Abel's contentment is
invaded by Gitelle's cancer and the broken relationship between Gitelle and Isaac.
While Abel's life unravels at the end of the book, the lives of Isaac and Avrom resolve. Their journeys were difficult, even heart wrenching, but at the end I believe that they came to terms with themselves and evolved into thoughtful, sensitive, balanced characters. On the other hand, Hugo and Magnus were flawed to the core and could never find peace.
One more thought. I would suggest that Abel's isolation of work and faith as the only essential ingredients of happiness is simplistic to the point of absurdity. But I also believe that one's work, which is so important to self-respect and fulfillment, is an essential component of happiness. Tragically, the work that is open to so many provides no opportunity for self-expression, creativity, mental stimulation. Abel was a craftsman who clearly derived satisfaction and contentment from his work. If Abel's life had been spent on an assembly line or flipping burgers (or the equivalent in 1930's South Africa) I suspect that Abel would not have defined happiness through his work.