Use "Don't we all..." in this situation:

• Someone talks about something that they wish for: "I wish..."
• You wish for the same thing, and you think that a lot of other people do as well.

The meaning of the phrase is something like "Yes, I wish for that too, but it's just a dream. It's not likely to come true."

For example:

A: I wish I could meet a guy who's responsible and doesn't act like an immature child.

B: Don't we all. Don't we all.

(Sometimes people will repeat this phrase.)