It is generally accepted that all the vertebrates and some of the invertebrates (those with large neural ganglia such as the cephalopods) are capable of subjective experiences. Amongst those experiences are the subjective, affective states, sometimes called 'feelings' or 'emotions'. The strong negative feelings are often lumped together as 'suffering' and the positive feelings as 'pleasure'. I have argued for many years that animal welfare/well-being is completely dependent on what the animal feels (Duncan, 1993, 1996, 2002). An animal's well-being is decreased by experiencing suffering and increased by experiencing pleasure. It's as simple as that.
Duncan, Ian J. H., "Distress or Suffering: What Should be Measured to Determine Animal Well-Being?" (2009). SEN. 13.