It can come suddenly, it can creep up on you.  Sometimes it is lurking just behind the next corner.  It is sadness.  It is that feeling that deflates you and takes the wind out of your sail.  It is strange what can do it toyou. It can be a thought, a smell, or something visual or auditory.  The cues that can cause that feeling are different.  To combat this you have to be in a positive frame of mind.  I try to to fill my head with positive thoughts, inspirational thoughts or music.  This sometimes helps me banish the sadness!