I find it amusing how confidently you state that. As if completely ignoring all the people who have said that same thing every year for the past 5.
That sounds a bit hand-wavey / crystal ball / technical analysis, but has reasons in fundamentals: Several years of collective prosperity decrease risk aversion, low inflation / money targets (thus interest rates) make (over) investment cheap (offices, tall buildings, capital stock, etc), collective memory fades.
Certainly not written in stone, however. A book recommendation covering the above, and influence of such fundamentals in long term financial trends: Irrational Exuberance by Robert J Shiller.