Well, nostalgia... What is this?

Why of all things do we have that one thing, period of time, smell, taste, etc that reminds us of a period in time when we felt was better than another?  Well, I am not quite sure how to answer that for someone.  There is no clear-cut answer but, you can walk with me through the 90s!  So, that's what we are going to do... Walk and experience this trainwreck of an episode as the very well-known phenomenon embraces us but is uniquely different for anyone who experiences it!