I am surprised how little attention is given to systematic counter-factual thinking in investment circles, relative to its use in other fields like project evaluation (my area). The references that are made above seem to focus on the potential negative effects of doing so. I think some of your readers might find it useful to try doing the following occasional analysis of a portfolio of stocks that have been bought and sold in a given period:
1. Profit/loss on a,b, c,d,e, stocks if all bought and sold as actually happened
2. Prodit/loss on a,b,c,d, e stocks if all bought as actually happened but not sold as actually happened
3. Profit/loss on a,b,c,d,e stocks if none bought as actually happened and all bought as actually happened
4. Profit/loss on a,b,c,d,e stocks if none bought or sold as actually happened

The 2nd, 3rd and 4th analyses cover three different types of counterfactual,whose outcome can be compared to the outcome of the real event covered in the first analysis.

My experience suggests that this can be a salutary experience, and for the time being this inclines me to trade less, not more. But perhaps in different market conditions this kind of counterfactual analysis might lead me in a different direction. Perhaps others could help us explore these possibilities...