This, which you said at the very beginning is kind of wrong anyway, I personally don't think it's out of place or a taboo for a trader to loose one or two trade in the course of trading, but like you later said in the body of your text, loosing in trade can become a problem when the trader continues to loose trades on a regular basis.
When loses in trade becomes a regular thing, then I think its time for that trader to take a break and go back to the drawing board to fish out what the problem might be.
Lack of or not enough knowledge of trading can be one of the major reasons a trader can constantly loose trades, but this is not the only reason, there are several other reasons or factors that can contribute to a trader constantly loosing trade, and one of those can be emotions - emotions from personal or private issues, emotions from family matters and so on.
It's up to the trader to sit down, find out what is causing the constant loss in trade and fix it in other to prevent further losses in his or her future trades.