Befriend a Dust Bunny?

I was also confused for quite some time, 'cause the hoverover tooltip on the greyed out 'Befriend' kept talking about how you can have only one dust bunny friend per floor, and my sims hadn't befriended any, and one of them had been on very high happiness for a long time. Regardless, eventually Befriend option became available after a lot of petting and feeding dust, so don't be discouraged by the confusing hoverover text!