What is it that no man wants to have

Question: What is it that no man wants, but no man wants to lose?

Question: What do you call a sad bird?

Question: What can be broken but never forgotten?

Question: Where do all the letters sleep?

Question: What tastes better than it smells?

Question: People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?