New Puppy Syndrome
When a person just out of a relationship jumps into a new relationship quickly. Similar to the situation in which a beloved dog dies, so the person gets a new puppy to replace the lost dog.
"Wow, she didn't take long to find a new man, it's only been a week since her ex left her."
"Yeah, she definitely has New Puppy Syndrome."
"Yeah, she definitely has New Puppy Syndrome."