I often select stories because somebody made a positive comment, the synopsis sounds interesting or the story is a result of a search. If it is a serial I open the part, which is at hand and if I like it I go back to the beginning.
The longest serials I already read. Sometimes I remember a specific detail in a story or see similarities to other stories or forgot about the previous content when I continue to read a serial after a long pause and therefore go back to reread some parts. For somebody who doesn't know this, it may look as if I'm arbitrarily jumping through the stories.
And finally I always read the end first! Otherwise I can't concentrate on the story and the details as I can't bear too much tension. If I know how the story ends it is much more relaxing to read and enjoy it. Btw. a few weeks ago I read a scientific study about the effect of spoilers. They found out that more readers who had seen a spoiler of the story before reading it valued the story higher, enjoyed it more and remembered more details than readers who didn't know the spoiler.
I would like to recommend Daniel Pennac's novel: "Comme un roman" to all authors. (In English it's: "Reads like a novel"). Therein he states ten inalienable rights of the reader:
1. The right to not read,
2. The right to skip pages,
3. The right to not finish a book,
4. The right to reread,
5. The right to read anything,
6. The right to mistake a book for real life,
7. The right to read anywhere,
8. The right to sample and steal ("grappiller")
9. The right to read out-loud, and,
10. The right to be silent.
Illustrated here:
http://www.walker.co.uk/UserFiles/file/Rights%20of%20the%20reader/NYOR_ROTR.pdf