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But also once a company starts to trade on what is known as "High Multiples", that is the Price to Earnings ratio is very high compared to the market sector the share is in, then investors start to become less prepared to buy the share.
High Multiples means the market is factoring future growth of SP and earnings. Which is why a positive announcement by a company can sometime mean the share drops because the market has already factored in the growth plus extra.
Investopedia is a great place to find free educational info on share trading and investment