"a wet blanket" is an old fashioned, informal expression which describes a person whose low spirits or lack of enthusiasm have a depressing effect on others. Another similar term is " a spoilsport" or " a wuss" Example: Come on, let's go out tonight and stop being such a wet blanket/spoilsport/wuss. Who cares if it's raining?

"A wet blanket" is a metaphorical term used to describe somebody whose negative, non-cooperative, perhaps sulking, attitude spoils the fun his/her friends/colleagues,... are having or about to have. A similar expression is "a spoilport", but the former is more descriptive, in my view. Just imagine yourself in a cold place urgently needing a warm blanket and being offered a wet, sticky, cold one. Very unpleasant, don´t you think? So try never to be a wet blanket/spoilsport. ("Don´t be a wetblanket"; you may also hear "What a wet blanket you are!) At bottom, the ethical/aesthetical basisfor the expression is: Enjoy life and let others enjoy theirs :-)