Pet hearts?

I'm making a pet shelter, I'm really loving the different curve balls I keep getting. I've adopted a cat called Shiloh and another called Mayor Whiskers. I noticed when they were together they were interacting but only after a little while of living together. Sometimes they clean eachother and I see little lovehearts.

I adopted a stray dog called Maddie- she was sick and of course, as a true shelter should, I adopted her and took her to the vet- my question is, she randomly barks and lovehearts pop up over her but she's by herself when she does it, any ideas what the hearts are? I thought it might be two pets falling in love but since she's the only dog i'm not sure.