The truth and lies about forex trends

It is probably the most common adage in forex, "the trend is your friend". Most of the prominent and respected authors and teachers promote the concept of trend trading, following the trend, buying when the price grows and selling when it falls. Many traders however, find it frustrating when applying this concept in practice. They enter the market on a percieved trend and find it reverses and hits their stop loss, only to then restablish itself soon after. So what causes this