Scientists give a theory about why deja vu happens. http://www.expatica.com/source/site_article.asp?subchannel_id=26&story_id=16910&name=Deja+vu?+Scientists+have+the+answer
There's more whats being answered in here than whys. While I believe a good number of deja vus are a result of remembering dreams, who's to say those dreams might not be our consciousness, or soul, going into the future? I actually had a sense of deja vu today - I was just leaving work when some of the events that happened today at work hit me as being familiar. I believe I had a dream about it some long time ago, but who knows, that dream may have been soul time travel.