I think because Guido saw Petrie as someone like him; someone who didn't fit in. When Petrie started to do well with his brothers and sisters, he started to fit in with them and I suppose Guido felt like he was on his own again. That and he probably felt lonesome because at least Petrie had siblings when he didn't know his family, or... forgot about them.
If not that, it could have been with happiness for Petrie because he wasn't like Guido and was starting to fit in. I'm sure Guido wouldn't want to wish his lonely "one of a kind" lifestyle upon anyone.

I'm not too sure, though. I haven't seen 12 in a while. :unsure: