People are social
People are social. We like to show off, to talk, to be heard, and by extension to share. There's every sign that "look at me" is a core human drive, along with its kissing cousin "look at this".
This sounds like an obvious thing, but then a number of people turn around and wonder why there are so many people using LiveJournal (and in a prior era, making homepages on GeoCities). The former explains the latter: we blog because people look, or they may look.
(And I suspect we often stop blogging because people don't look. This puts the LiveJournal drama over friendslists, friending, and defriending in an interesting light. Is the much-derided friendslist actually a crucial ingredient in LJ's success?)
There's another aspect of this: open source. People often wonder what drives people to work in open source, but I can think of a simple explanation: "look at this". People show off what they create (ask any parent with kids), and programmers create programs.
(This leads to a neat explanation of a lot of drama around credit, open source project ownership, and so on. You're not showing off if your name's not on people's minds. Also to consider: are developers attracted to popular projects as a way of more efficiently showing off?)
A corollary: for many programmers, the people they are showing off to is their fellow programmers, not the users of their programs.
(I doubt any of this is original to me; I just feel like putting it down right now.)