[ElProlactin]

> If you did your due diligence and you believe that this particular asset will grow within 5 years...

This assumes that most people know how to do "due diligence" and that their "predictions" are accurate. Most people don't actually have the knowledge and skill to evaluate the investment vehicles (stocks, bonds, etc.) available to them so their predictions are inherently limited and flawed.

> ... when it starts dipping after few months, and nothing major has changed in your predictions - you should buy more instead of selling.

One of the biggest mistakes average people make is selecting investments with risk profiles and durations that are mismatched to their needs and objectives. This is why, for most people, it's much better to use a properly-selected model portfolio than to try to pick individual stocks.