The analogy with stealing is not correct. The ethical issue is somewhere else — see the reference above in my previous reply: http://doctoralwriting.wordpress.com/2012/09/13/recycling-old-papers-and-self-plagiarism-is-it-a-sin/.
Self-plagiarism in its worst form spins the same information with practically identical words, wasting the reader’s time with no net increase in knowledge perhaps benefiting only the author and the subsequent publisher(s). Spinning the same information with different words is almost as bad. There are certain exceptions especially in the sciences. For example, there are not too many ways of writing certain equations and we are all familiar with those ways. There is no plagiarism or self-plagiarism in this case.
Another exception is when one is writing a review paper. In this case, chunks of writing (hopefully not large chunks) may be quoted verbatim but should be properly attributed.
One awkward byproduct of self-plagiarism even though the commentator felt that it is not stealing is that the copyright has generally been assigned to the publisher. Publishers usually do not press this issue but legally the work is now theirs. On the other hand, authors of journal articles allow these publishers to make money at their expense!
