I had one cat like that (extreme agitation and threat displays through the window at stray cats). I have another one who, after his cat companion died, started

I had one cat like that (extreme agitation and threat displays through the window at stray cats). I have another one who, after his cat companion died, started exhibiting play aggression and increased clingy behavior toward me. When I got him a new cat companion (taking care to choose one that enjoyed rough-and-tumble play with other cats), his behavior returned to normal. These two do seem to actually enjoy their daily contact.