Other forms: puppy loves
The crush you have on the girl in your fifth grade math class probably won't last forever — in fact, it's almost certainly just puppy love — when two very young people love each other briefly.
You can use the term puppy love to talk about love between kids or adolescents, or about a very short-term infatuation, or even a one-sided crush. Sometimes people use it in a derogatory or dismissive way when they don't take a relationship seriously: "She says she loves him, but it's just puppy love." Think about the intensity and enthusiasm of a puppy's affection, and you'll understand where this phrase comes from.