Friend or Foe

My 3 1/2 year old lab Freya an my latests addition Fred who is now 5 months old. They are best mates one minute then the next they are chasing each other and playing/fighting. Freya puts his head in her mouth but he is not as gentle as she is as he sinks his teeth in more often than not! Yet she still does back for more!! Silly girl doesn't know when to stop.