How do we read?

Who decides that we should start at the beginning and work our way through to the end of a book.
I am reading a John Irving novel at the moment (Prayer for Owen Meany) and because I was also reading the new Harry Potter book, I thought I'd skip to the end of Owen and see if I should keep reading.
The good thing about reading the end first in this particular novel, is that it's giving me a greater appreciation of the craft of writing - how JI is leading the reader to the end.
I still have a long way to go - and so far I've read the end three times (and cried each one time) - and I'm more determined than ever to keep reading to see how he constructs the journey.
Surprisingly, when I mentioned the notion of not reading a book from start to finish, one of my mates was horrified and aghast - as if to do such a thing was akin to some kind of crime. Maybe she's right - but it's a relatively non-dangerous way to take a walk on the wild side!