DTers Weekend Aftermarket lounge 13-16 July, page-99

I'm currently rereading Trend Following by Michael Covel and he would say that one of the ways traders fail is not letting trends run to the end. On the trend following model the bulk of profits are made from a relatively small number of trades, and there are a large number of losing trades which get sold early to minimize losses.
The trouble with trading is that our psychology urges us to do exactly the wrong thing. Take profits early so we don't lose them and keep losers in the hope that they will come back.
It may be the case that day trading has a different set of rules, but I suspect that the principle applies in all time frames.