It's not all rational - for those stocks which are up a lot,...

It's not all rational - for those stocks which are up a lot, people are excited or even religious that any level of rise is justified (even beyond the fair value, most still hold and prey for even higher SP), any weak side of the story is ignored. If they all hold and only keep adding in the short term, SP is more likely to stay high, and the fanatic sees that SP rising in-turn ‘proves’ his/her belief, and it became self-fulfilling.

On the other side, for those undervalued ones, you have SP being lower and those fanatics lost interest on momentum shortage – many investors who is rational is likely then in this sort of stock and will sell if the price rise to where they see fits, so any rise is questioned and likely partially profit-taken, i.e. they are not holding forever with religious but doing the rational action – the momentum will then be hard to build. Once you lost momentum, and happen to burn a few fanatics entered at wrong price, they quickly turn to the anti-fanatic and will attack on anything on the stock they previously owned (it must be a shitty stock, and I was smart to sell it; but if I bought it back later, I must be smarter – you can easily tell who sold and bought back from posts)

I will not pump FFX like the fanatic by adding rosy comments here, but you need to understand in the long term, even fanatic will need to sell the holdings for other opportunities or real life expenditure (it became a question into who sell first or sell before the momentum got ended or that entire sector turning from loved to unloved, all performing sector will eventually get a sell-off and rebalance which only strong ones with real business survived) While the SP can stay low on undervalued stock, but if the stock is truly undervalued instead of a wet dream that might not eventuate, then the net earnings will always support SP anyway (dividend vs buy-back vs got outright take-over bid etc)

Good luck to everyone regardless of if you are buying/holding/selling/not holding, we are all here to make money, do what suits our timeframe and trading style and it's very natural to have different opinions on the same piece of info.