Souls can only incarnate inoto bodies with DNA responsive to and compatible with that soul.
Because souls are shaped by experiences and thoughts, different soul types will reincarnate into different body types of the same species. As humans, their personality is influenced and restricted by the characteristics of their soul. Spiritual evolution is limited by one's biological, genetic, and physical composition. For instance, a dog cannot grow emotionally as we do because their brains are biologically incapable of perceiving the same range of emotions. Dogs also do not feel equal self sacrifice and devotion to truth that some of us humans enjoy, and we are therefore superior to them.