What goes up does come down … hills, moods, all manner of things, and it is not simply gravity at work.  It is the Law of Balance.  Were it not for this Law of Being, you would experience chaos in your world of duality.  Pleasure is balanced by pain so that addictions do not run amok.  Oh yes, the human mind wants to nitpick and give examples, and ask, “But why?  It is not fair!”  And fairness is precisely in the middle as a result of the law of balance.  You do not care for the drastic ups and downs?  Then practice moderation in all you do.  We love you so.