User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-1359322-20150309103939/@comment-9595671-20150309110506

Either that or Juvia still feels guilty over killing Gray's dad

Or its possible over the past year Gray's changed, and if she's sad over it then he hasn't changed in a good way. Since he wants to beat END it wouldn't be surprising if he let that goal change him.

I've seen a lot of stories where a single powerful goal can change people, for better... or worse.

It could just be that with Fairy Tail disbanded she's sad cause her friends all went different ways and Gray went his own path too. In other words she could just be feeling really lonely right now.