Hair gets its natural color from melanin, the same pigment that lends hues to eyes and skin. Two kinds of melanin (eumelanin and pheomelanin) combine in different ratios to produce different shades. More eumelanin makes hair dark (black hair is almost entirely eumelanin), and more pheomelanin makes hair red (though a bit of pheomelanin mixed with eumelanin is what makes brown). Blond hair can have any combination of the two, but in lower levels. So, having blond hair is sort of like turning down the volume of another shade.
When hair totally loses its melanin, it looks white or gray because of the way light moves through it -- in the same way that melanin-free eyes are blue, not clear. (Fun fact: Polar bears have "white" hair because their fur is actually clear.)
Permanent artificial hair color doesn't just throw pigment onto your hair (though more temporary dyes, like Manic Panic, do just that, which is why you need to bleach first) but instead open up the shaft, break down the natural pigment and slip in some molecules that (once combined inside the hair) make a particular color.
So when you want to change your hair color to something lighter than natural -- either by going platinum or just shifting shades -- the original pigments have got to go. Bleach works by going into the hair shaft and reacting with the stable pigment molecules, breaking them down into components that will wash right out of your hair and down the drain.
But when it does that, it also breaks down the natural fatty acids on the hair shaft, weakening the strand. This is permanent damage, and the longer you bleach the worse it gets.
How to protect your new 'do? Bleach your hair as little as possible (duh). When it's time to touch up the roots, only bleach the roots. Bleach damage is as cumulative as it is permanent, and your ends will be less equipped to survive it every time. Avoid excess brushing and harsh shampoos.
There are a lot of products that claim to restore health to bleached hair, but the C&EN video recommends those containing ceramides, which are fatty acids not so different from the ones your bleaching regimen will obliterate.
Keep in mind, however, that these fatty acids will at best just coat your hair temporarily. Your tresses won't be restored to their pre-bleaching health, but you can make them look a little less fried and protect them from further damage.