In The Call of the Wild, loyalty is rare, but strong, when it exists. It is forged by the extremities of circumstance (Thornton saved Buck from death) and repaid with similar intensity (Buck saves Thornton repeatedly). The big question in this novel is whether Buck has greater loyalty to the wild and his own feral nature or to the man that has saved his life. Buck is unable to commit to his life in the wild until Thornton dies, perhaps suggesting that his love and loyalty for the man are stronger than his ancestral urges.
Buck's love for Thornton develops while he is adapting to the wild, but this love is incompatible with the natural world because of its domestic nature.
The difference between love and loyalty is illustrated in the contrast between the loyalty Buck feels to François and Perrault as opposed to the love he feels for Thornton.