Ivan notes that Mack grew sullen. What had caused Mack to feel that way?

Mack grew sullen because he was sulking.... he enjoyed curling up with Ivan at night and sleeping, but as Ivan grew... cuddling became impossible, as Ivan was far too big to fit in a chair or a bed.

Mack grew sullen. I grew bigger. I became what I was meant to be, too large for chairs, too strong for hugs, too big for human life.