Your Old Favorite Novels Are Now Awful
A side effect of that side effect is that all that extra reading with a newly learned eye takes away a bit of the magic of books, and at minimum will make you a far harsher critic. Once you've sunk your teeth into writing a bit, learned some of the common pitfalls, like overusing cliches, and busted your ass to send them packing, they'll stick out like a sore thumb when you see them in other people's writing. The same applies to stock characterizations or clumsy prose or any other flaws that you might have once sailed past.
Rereading authors you once loved is a particularly enlightening experience. The thriller and adventure and fantasy novels I once read now look awful to me. These books were once a pretty enjoyable part of my adolescence, and that they've been forever ruined by my growing brain is bittersweet, to say the least.
"Also, wrestling was fake, Santa was your dad, and your dad was an actor paid for by the scientists who actually bred you."