I also don't think this is unique to polyamory. Monogamous people compare current lovers to previous ones. Sometimes you feel threatened when you meet an ex who is brilliant, handsome, charming... and you wonder "why did she leave him? What if she goes back with him?" The nice thing about poly is, they're still seeing that person and despite whatever he's got going for him, she still decided she wants to see you too. So you remove that threat of "maybe she'll leave me to get back with him."
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker