I think they all want to be friends but the hissing and spitting is a ritual they have to do first to get acquainted and set the rule of who is in charge.

Ever notice how when there is a new one in the house, they just can NOT stay away from that door (mine do that).

When I took my babies to Kevlins the last time I was getting ready to go back to work, Lucky was shut up in the bathroom....this is when he was so sick.... Amy would not stay away from that door and she would just sit there and HOWL!! The only time she has ever howled before is in the car but she kept going back to that door and howling!