忘れてて良いのに
I understand this as: "You should forget about it"
But what does のに do?
のに is a conjunction that corresponds to "even though ~" or "despite ~".
(clause A)のに(clause B)
= although A, B
.
Unlike けど, it often (but not always) is followed by something regrettable/disappointing. Examples on JGram.
The latter clause is often omitted, and it implies the reality is something regrettable and contrary to the Clause A.
~のに。
= although ~, (something contrary)
, A, but...
, I wish ~
.
So 忘れてて良いのに means "You could have left it (although, in reality, you recalled and mentioned it)".
Examples:
- こんなに美味しいのに。 It's really delicious (and I wonder why you don't want to eat it)!
- そんなに賢いのに。 You are such a smart boy (and I don't understand why you did such a silly thing)!
- もっと頑張れば良かったのに。 You should have worked harder.
- 空を飛べたらいいのに。 I wish I could fly.
See also:
I can suppose the following situation.
A helped B out of his (her) hard situation.
B is one of the best A's friends, and A wants to be kind without being patronizing.
However, every time when B meets A, B shows his (her) thanks.
Then A says "忘れていいのに"
The full sentence could be "まだあなたは感謝を忘れないんですね。私は忘れていいと思っているのに(どうぞ忘れてください)。" = "You still owe a huge debt to it、but I sincerely want have you forget it."
"忘れていいのに・・・" implys such nuans.