Why I lost money in some stocks in the stock market?

Why I lost money in some stocks in the stock market?

Buy low, sell high = GAIN
Buy high, sell low = LOSS

Factors to consider:
Price
Company

Price
If you can have the intellect and the emotional control to buy low, you have already WON most of the time, with a high probability. (I often quote, "many a sin is forgiven when you manage to buy at a low price.)

To be able to buy a share at a low price, means you have the ability to value this share. Valuation maybe based on assets, cash flow or multiples of earnings, book etc. Do not over-project growth in your valuation. You can only be approximately right in your valuation. This is alright, as long as you are not absolutely wrong in your valuation. There are also many qualitative factors that cannot be quantified in the valuation.

At a certain price, the stock is undervalued, at another it is fairly priced or overvalued. The ability to value a stock is the most important knowledge of a value investor.


Company
Choosing the right company to invest into is very important for someone who has the buy and hold for the long term philosophy. Choose the company in a business with durable competitive advantage. Its business moat is so huge and deep, that competitors find hard to erode their growth and profits over the long haul. Therefore, looking at the quality of the company's business and the management (integrity, intelligence and hard working) are important here.

Even the best company can fail. Look at the Dow Jones Index of 30 companies over the century. The index is made up of the best companies of each period. Many have faded or disappeared into oblivion. Of the 30 original companies at the start of the last century, only 1 or 2 of these are still in the Dow Jones index.

So, you may lose money when your high quality company with good management that you bought at undervalued price deteriorated in its business fundamentals permanently. In this situation, you will need to sell urgently to minimise your loss.


Conclusion
If you have done the hard work in selecting good quality company and valuing this company, the chances of losing money are few. From the above discussion, essentially, these are: (1) buying a poor quality company with no durable competitive advantage, and (2) paying a high price for your purchase.

This approach essentially means focusing on valuation and the company, and is not influenced by the market or herd mentality. The market is there to be taken advantage of, when the prices are right to buy or to sell, and otherwise for most of the time, can be ignored.