According to ChefKeith my dog Jack howls and carries on when I leave the house even though he's not alone.
He is a rescue dog and probably had a horrible life until I found him. Therefore, he may be more insecure thinking he's being abandoned again. He has a lot of quirks including being afraid of boxes (maybe thinking we're moving again or something)
My little cocker mix showed up as a half-grown pup, starved and skittish... throughout her 16 year life, she'd start to throw up and act stressed when I was packing. I had her in college, moving back and forth between school and home a couple times a year, so it happened a lot.