Diego-san and Theo have always had a complex relationship. I think Diego tends to look at Theo as someone he has to tolerate rather than enjoy having around.
I have noticed in the past couple of weeks however that Diego doesn't automatically just leave if Theo is in the vicinity. Indeed it's becoming more common that the two will lie next to each other.
Of course the full speed chases up and down stairs, around and through furniture, continue to the detriment of the detente.