When someone says, "I can't wait," the proper way to reply would be "Me neither."
"Me neither" is used when you agree with a negative statement or express the same lack of enthusiasm or anticipation about something as the other person. In this case, you both share the sentiment of not being able to wait.
Example: Person A: "I can't wait for the weekend!" Person B: "Me neither!"
On the other hand, if you are expressing the opposite sentiment, where you are excited and looking forward to something, you would say "Me either."
Example: Person A: "I can't wait for the weekend!" Person B: "Oh, really? Well, me either! I'm really excited for it."
So, in summary:
- If you agree with the lack of enthusiasm (both can't wait), say "Me neither."
- If you disagree and have a different feeling (you can wait or are excited), say "Me either."