Life is pain, as someone smarter than us once noted, and for White Fang, it means a lot of pain. Characters suffer so they don't have to die, they suffer while competing and they suffer as a part of nature's grand design. It stinks, but there's no way around it. For White Fang, at least, his suffering eventually ends: and not in the grim downer of a way that it ends for so many other people. London really wants us to know what a happy ending feels like, and he can't write one for White Fang unless his poor little puppy dog goes through the ringer first.
Suffering ultimately makes White Fang a better pet than he might be if he'd lived a cushy life all along.
Suffering doesn't do anything good for White Fang; it's only damage that needs to be undone.