Well Benvolio believes this because Romeo has an immature love in which he swears to never forget Rosaline and yet he unwittingly does once he sees Juliet. So he only remembers the girls he currently sees, and forgets all others. Plus he doesn't even know Juliet at first, so his blind love is rather reckless. And a reckless person like Romeo would surely be unmoved by what benvolio said.
