There are probably a million reasons. Here's one.
At times I read something and feel that the words have touched a chord somewhere. They've spelt out a piece of life as I've lived it, described a thing I'd never been able to put into words.
A feeling/emotion/idea/outlook/something lives somewhere deep inside you, beyond the realm of language. Perhaps in some remote atavistic corner of the soul. Something felt , never defined.
And then you read a book, perhaps a story of someone generations apart, continents apart. But the author has described that little thing, just as if they had dug into the recesses of your being, and captured that ineffable feeling with both hands.
The realisation can be even more startling with poetry - the feeling of - hey wait, that's me! that's my state of mind! How did someone else write it out so well?
Its an amazing feeling, at once both exhilarating, and reassuring. To be able to understand yourself better, and also to feel one with this world.