Dog no longer attention seeking

I'm going to try to not make this a saga but still provide all relevant information.

In 2014 we adopted a nearly two year old pug mix. She was a loving, very well-behaved dog, but absolutely could not handle being home alone.

After taking her to meet dozens of dogs we found one she liked with the only negative being that he was an older (eight years old) dog. He was an amazing dog too. The yin to her yang. Sadly we didn't get to have him very long, about six years before he passed.

The pug mix's separation anxiety came roaring back and we found her another dog, a border collie mix. The new dog is also wonderful, but the neediest dog ever. It was cute-needy, but still a lot.

Recently we said goodbye to the pug mix and suddenly the border collie mix is significantly less needy. Generally he still likes to be in the same room as us, but he'll spend more time outside on his own (doggie door) or napping in a chair by himself than he did before.

We are relieved that he's not desperately needing a companion because we are still gutted by losing the pug mix and didn't want to have to get a new dog, but we're also perplexed by the change in behavior.

We did an at-home euthanasia so he knows the pug mix is not coming back. Is he less needy because there's no competition? Is he grieving too? Does he just like being the only dog? I really wish I knew what was happening in his cute little head and wanted to hear other people's thoughts/experiences.

Thank you!