This is something I didn't realise wasn't true until I read Harold McGee's classic 'On Food and Cooking'. Most of us have read cookbooks or watched videos in which a cook confidently promises us that browning a piece of meat will 'seal in the juice'. This sounds plausible, because we all know that a well-browned steak or chop has an almost crispy crust; surely that thing is somehow protecting the wonderful juices inside from escaping? Wrong, wrong, wrong, friends. That crust is partly MADE of the wonderful juices, which are being squeezed from the meat by the heat that's causing the muscle fibres to contract. As long as your food is on the fire, the heat is making it shrink, which in turn is making it squeeze itself like a sponge, and as soon as the juices inside reach the outside and encounter the hot pan, as long as the pan is hot enough the sugar in the juices will caramelise and cause the browning reaction that those of us who like that kind of thing long for. If this all sounds very carnivorous, vegetarians should consider that it also goes for vegetables; something very similar happens when the sugar in onions caramelises and gives you nicely browned fried onions, and likewise when you fry mushrooms, and I mean fry them properly, not just cook them until they sweat grey juice into the pan.
If you think about the 'sealing-in-the-juice' notion, you soon realise that it makes no sense. If browning a steak truly sealed the juices inside the meat, then if you kept on cooking the steak it would eventually explode like a gigantic piece of popcorn, because the juice inside would boil and the resulting pressure would rip the steak apart. But we all know that if you keep on cooking a steak forever, eventually it will turn into the sole of a shoe; the juices will be squeezed to the outside of the steak, where they will partly caramelise and partly evaporate, and the meat will dry up, and the steak will become (IMO) inedible.
The truth is, placing food on a very hot surface is liable to cause a browning reaction but it will not 'seal' the food in any way. The art of making sure that food stays juicy is trickier than that; for example, it's one of the reasons why meaty stews and casseroles are always tastier if you cook them the night before you eat them. Why? Because if you cook them and eat them in one go, the long cooking process will mean that the meat has been exuding its juice into the casserole all evening, and is liable to be a bit dry. If, however, you stop cooking it, let it get cold and then gently reheat it the next day, as the meat gets cold in the liquor it starts to reabsorb all the liquid that it was being cooked in, so it becomes juicy all over again, but more interestingly so (because the liquid is flavoured with the vegetables and herbs, etc. that you threw into the pot). If you then gently reheat it the next night, the meat doesn't have a chance to shrink and squeeze out all that great flavour, so it remains juicy. Isn't science wonderful? (There may also be some minor fermentation/decomposition things going on that help to enhance the flavour of a reheated casserole.)
Curiously enough, chicken seldom seems to be better for being reheated, unless you're brave/insane enough to make genuine coq au vin. One day, I will make this and report.