Because he's always had her back, and she'll always have his.
Because he knows what she smells like (falafels, ocean and shea butter).
Because "business as usual lost all meaning" when he thought she was dead.
Because their very first conversation was about phone sex!!
Because they feel the need to be physically close to each other all the time.
Because they have amazing chemistry.
Because they can survive anything.
Because after everything - Jeanne, Rivkin, Somalia - they always come back to each other, and their bond is stronger than ever before.

Love them, Piper! :)

Because he thinks her "contents....priceless"
Because everytime she's over him, that's so not his knee...