Overconfidence is also a major factor in trading too frequently for the retail investor. Round trip (a buy & sell) trading costs that include broker fees and the bid/ask spread are expensive but only part of the problem. Even more importantly is the temptation to time the market, and may result in missing out on big moves.
We also tend to rely too much on our familiarity with one or more companies only to find out that its a great company, but a poor choice for an investment. We've worked at Company ABC for years, and it really is a good company to work for, but maybe not so much to overload in our portfolio.