ExtremeProactivity is what is often called "courage" or "aggressiveness" of "fearlessness" in XP. It is one of the ExtremePrinciples XP uses to achieve its goal to EmbraceChangeContinuously.
I tend to categorize the various ways to respond to change (EmbraceChange) as follows:
Some might consider YouArentGonnaNeedIt (YAGNI) as an example where XP recommends being reactive over being proactive. I would disagree. I think YAGNI is an example of ExtremeProactivity tempered by ContinuousSimplicity. YAGNI is talking about waiting to add something to the design because it is more of a "possible want" rather than a "definite need" (see ExtremeNeed). There's no problem yet, just a possibility that might or might not turn into a need someday. CollectiveCodeOwnership tells you to fix a problem yourself when you see it; it does not tell you to add something to fulfill a need that hasnt been asked for yet. So I would say YAGNI is about being extremely proactive with regard to simplicity!