Tom: Chris said your son is dating?
Carol: That's what he's telling everyone.
Tom: I thought he was only 13!
Carol: He is. It's only puppy love. But he and his girlfriend are so cute together.
Tom: I remember my first girlfriend. We were both 12, and her name was... actually, I don't remember her name.
Carol: Don't tell my son that. He told me he's going to marry this girl one day.
Meaning: The expression "puppy love" refers to the strong feelings at the beginning of a relationship, often used for young people and implying that the relationship won't last. Here, Carol says that her 13-year-old son's relationship is only "puppy love," which Tom seems to agree with when he can't remember the name of his girlfriend when he was 12.
Sheila met her husband in high school; they were both only 17. Their parents and friends said it was just puppy love, and that they'd never last. But they got married 3 years later, and they have been happily married for 20 years now.
Meaning: The expression "puppy love" is also often used in this context, referring to relationships that started at a young age but developed into something long term.