In One Piece, why doesn't Robin tell Luffy what had happened during her training?

Question

Why didn't Robin tell Luffy what had happened during her training since she met Dragon, Luffy's dad.

Also meeting,

Sabo

In the Dressrosa arc.

It seems odd that no one else in the crew shared their stories with each other.

Answer 1

Since the strawhat Pirates reunited, they have been on a rollercoaster ride of events. In the One Piece universe, not much more than a few days or maybe a week has passed and they just did not have time to "catch up". They always have had bigger things to worry about. The thing is, suppose they would have had the time, it actually makes a lot of sense for Robin to keep quiet about her whereabouts and it quite fits her character as well. The following list are reasons why Robin should clearly not have told Luffy about Sabo.

• Robin did not know about the relationship Sabo, Ace and Luffy shared. She only knew Ace and Luffy were brothers.
• Even if Sabo did tell Robin, she did not know Luffy considered Sabo to be death.
• Even if Robin did know about all of this (which seems very unlikely), Sabo was to be considered death for the rest of the world too. So if anything, he probably wanted to keep it that way for the time being.
• Sabo might even have asked Robin to keep it a secret, to not spoil the surprise. He probably wanted to tell Luffy himself, similar to how Shanks did not want to meet Luffy during the Marineford War.
• Robin has been hiding her activities since she was just a child, so it was just natural for her, not to talk about it anyway.
• Even if Robin wanted to share her whereabouts, she was probably told not to do so by Dragon himself, because being associated with the Revolutionary Army has major consequences. The World Government might want to interrogate her about the Revolutionary Army as lots is still unknown about them as of chapter 787. They want to keep a low profile as long as possible.

Edit as per Masked Man's comment:

• Robin doesn't really "socialize" much, she is serious most of the time and doesn't get into "personal discussions" too much, so it would be totally out of character for her to talk for hours about what she did during the timeskip.

Answer 2

It's probably because Oda wanted the appearance of Sabo to be more of a "surprise" to the reader (not that it was particularly surprising in the first place).

If Robin had mentioned anything before that, then the payoff would have been lessened.