Markets are not rational. They're created by people. They're based on emotion. Emotions can rise, and emotions can fall. You can either ignore the emotion or take advantage of it, selling when times are flush and buying when times are hard.
I hold shares of both Google and Apple, but I haven't been adding to my Google holdings lately. I've bought more Apple. Because when everyone tells you nothing can go wrong, something always does.
At the time of publication the author had positions in YHOO, AAPL, MSFT and GOOG.