
In life, we are taught to take action. Don't sit on your bottom, looking at a problem, do something about it. Make a start and figure out the best solution as you go along. If your house is a mess, get busy fixing it. Don't procrastinate. If your paunch is spreading, pull on some tackies and get outside, etc, etc.
The problem is that when it comes to investing, the opposite is true - you need to have a bias towards inaction. You have to avoid responding to market chatter. You have to ignore silly headlines about market concentration, imminent corrections, credit concerns and the blathering of anti-market politicians.
Barry Ritholz said it well recently: "fiddling with your portfolio acts as a salve to reduce your anxiety, because it gives you the illusion of control. Doing something makes you feel like you have influence over random events, which you decidedly do not."
In general life, the best advice is "don't just sit there, do something", but when you are invested in high-quality blue-chip stocks, the best advice is "don't do something, just sit there."