An extreme example: You buy a house and then the value plummets to a fraction of what you paid. Why might you need to stay in that house? Other house prices presumably fell along with yours so you can still sell yours and buy a similar one. I think people find it painful to sell at a loss because it makes a loss on paper become explicit. If you don't sell then you can tell yourself that you're speculating -- you really think your house is worth more than it will currently fetch so it's imprudent to sell. But probably you're not smarter than the market in that regard. In other words, there's no harm in turning a loss on paper into an explicit loss.
Bottom line, if you want to move from house A to house B and the price difference (plus moving costs) is worth it, then do it, with no thought at all to the price you happened to have paid when you bought the first house.
Of course, moving costs when realtors are involved are huge so the original point partially stands. You're never stuck in the sense the parent implied, moving is just more expensive when you buy vs rent.