Strange bedfellows

I say strange not because they are cat and dog and those two just aren't supposed to get along.  They have an obviously close relationship in which he (the cat) will seek her out wherever she is and curl up beside her to sleep.  Reciprocally, she will often become quite agitated when he is outside and she perceives he may be in danger.  She barks to be allowed out and races ferociously in the direction of whatever threat she perceives is abroad.  He meanwhile sprints indoors like the Tam o'Shanter himself. 

Their relationship strikes me as strange because they can't play with each other.  When he wants to play he'll chew her tail, jump on her back, punch her head and generally try to goad her into retaliation.  When she does try to play, she'll take a swipe at him with a paw the size of his head and he runs away.  The ne'r shall meet on the playing field.