It is obvious that the performance of a stock last year or last month is no reason, per se, to either own it or to not own it now. It is obvious that an inability to "get even" in a security that has declined is of no importance. It is obvious that the inner warm glow that results from having held a winner last year is of no importance in making a decision as to whether it belongs in an optimum portfolio this year.
For some reason, people take their cues from price action rather than from values. What doesn't work is when you start doing things that you don't understand or because they worked last week for somebody else. The dumbest reason in the world to buy a stock is because it's going up.
If you have more than 120 or 130 I.Q. points, you can afford to give the rest away. You don't need extraordinary intelligence to succeed as an investor.
If you do smart things and use leverage and do one wrong thing along the way, it could wipe you out, because anything times zero is zero. But it's reinforcing when the people around you are doing it successfully, you're doing it successfully, and it's a lot like Cinderella at the ball.
At age 19, I read a book [The Intelligent Investor] and what I'm doing today, at age 76, is running things through the same thought process I learned from the book I read at 19.