From ThoughtfulReactionsToXp:
Second is something that CollectiveCodeOwnership actually helps foster and that's EgolessProgramming. That is, when somebody changes your code to improve the system you learn from it and appreciate it, not think that your idea was somehow bad or wrong. You understand that when you wrote it you did it the best way you could for a particular story. As new stories come along, more is learned and the system is adapted accordingly.
When we did the VcapsProject, we actually found everybody's pride and enthusiasm in the system go up dramatically. There weren't anymore comments like "I tried to do it right but this other crap that some Jangmeister (see JangIt) wrote just kept getting in the way, so I had to compromise". What you find is that when you need to add something you can add it the right way because the system is always in a well designed state (RefactorMercilessly, DoTheSimplestThingThatCouldPossiblyWork) and your pride in what you added is increased because you didn't have to compromise your design.
-- TomKubit
One other point to make, and I tried to illustrate it above, is that without some supporting practices CollectiveCodeOwnership might be difficult, at best. But having these practices in place actually makes collective ownership a natural (possibly necessary) addition to the XP practices. -- TK
One of the important supporting practices is a CodingStandard that the group agrees to (and all newcomers to the group have to either accept or go somewhere else). You can't spend time reformatting code or having difficulty reading it -- you need to be able to look at a piece of code and say "hmm, I wonder who wrote that... was it me?" instead of thinking, "Oh, crap, I better fix what's-his-name's indenting so I can read this, THEN I can refactor it."
Don't confuse collective ownership with no ownership.
This is an excellent point. When you practice collective code ownership, your pride is a little different, and is based on different things, but it's no less real. It can be a lot better. See comments on choral singing in AnalogiesFromMusic.