This show is just pure fun. It combines drama and pathos with witty and intelligent humor.
Putting the terrific Castle/Beckett relationship aside for a moment, the scenes between Castle and Esposito, even if short, are pure gold. Ditto for Castle and Alexis. She is a delight as the best teenager on television. She is mature and serious about her life, but she has enough of her father in her to be an absolute delight. Kudos to Molly Quinn. Rick asking Alexis, "But who's going to play with me?" was perfect.
As for Castle, he's mature and caring enough to have provided a great life for the family that he loves, but the part of him that won't grow up is wonderfully endearing and exactly the kind of man that I would find attractive.