Empathy seems more salient in humans than in other animals precisely because it is rare - it is normally suppressed by higher cognitive functions and manifests itself only sporadically.
Another point: empathy develops in children in a rather early stage of development, before the cognitive functions develop. This inability to 'reign-in' empathic emotions is, in a way, a defining feature of childhood, whereas adults are supposed to be less empathic and more rational. This further suggests that empathy is an 'early' evolutionary trait -i.e. one shared with 'lower' species' in relation to 'higher' cognitive functions. Moreover, it suggests that humans are more capable than animals to suppress their empathic feelings.
Wojtek