Although I realise a behavioural explanation might hurt long
established anthropocentric viewpoints, I believe the answer to
this question is better understood if one adopts an ethological
viewpoint. In ethology one describes tenderness as a way of
consolidating mating bonding. It is indeed a well documented
pattern in many evolved animal species that individuals in a
mating relationship with another individual of the same species
(partners) will regularly express tenderness as a means to
trigger reciprocal feelings, lower the partner's defence and
ultimately enhance the chances of a successful relationship.
When in "non mating mode", the standard relationship between two
individuals of a given species is dominated by competition for
existing resources (predation territory, food or other survival-
critical resource...). This is a situation where the relations
between these two individuals can be marked by aggression and
defence reflexes (Konrad Lorenz in his book "On Aggression" has
shown how aggression was an