Understanding Scala's self-typed traits

SelfTyped.scala

trait Huggable {
  self: Pet =>

  def hug() :Unit = println("Dawww...!")
}

class Pet

class Puppy extends Pet with Huggable

class Kitten extends Huggable
// Compilation error!
// [error] /home/noahlz/projects/prog-scala/src/main/scala/app/SelfTyped.scala:12: illegal inheritance;
// [error]  self-type app.Kitten does not conform to app.Huggable's selftype app.Huggable with app.Pet
// [error] class Kitten extends Huggable
// [error]                      ^
// [error] one error found

object SelfTypedMain extends App {
  new Puppy().hug
  new Kitten().hug
}