Anne states that a "good hearty laugh" would be therapeutic because all in inhabitants have become so very serious.  For, there is a mounting fear of being found out since the warehouseman, V.M., seems to be suspicious and Anne and the others fear he will find them in the "Secret Annexe."

A sensitive and intelligent girl, Anne is given to much self-analysis as well as analysis of what goes on around her--both of which cause her anxiety.  For instance, she worries about Kraler, who has sneaked up to them.  To avoid being heard as he leaves, Kraler has taken off his shoes and does not put them on until he is outside on the street:

Whatever would the people in the street have thought...Gosh!  the Manager in his socks!

The answer to this is that Anne says that a good laugh would do her more good than taking ten Valerian pills would.

She says this in a letter to Kitty, which is dated Thursday, 16 September, 1943.  Anne says that she is taking lots of Valerian pills to try to keep from being depressed.  However, she says, the pills do not really do any good.

The reason that Anne keeps getting depressed is the fact that everyone is developing something of "cabin fever."  They are getting tired of always being around each other and always being tense.