The rivalry of siblings is something experienced by nearly everyone. There are different dynamics, of course, and it can depend on a whole host of variables such as gender, age difference, and family environment. Not everyone has the same experience, but it probably safe to say that younger brothers have certain similar features. One in particular characteristic is mimicry.
The Diamondbacks are the little brother of the NL West, and baseball at large. Even if the fanbase composed of savvy fans, collectively we are looking to others for direction. Virtually every complaint of the Diamondbacks or its fans is a variant of "why can’t Arizona be like this?"
There are plenty of good ideas out there in baseball; they are traditions for a reason. A hot dog and a beer at a game is a wonderful thing, and who doesn’t love singing Take Me Out to the Ball Game during the 7th inning stretch? These are innocent manifestations of mimicry.
The other side of this is Arizona’s obsession with being taken seriously. Part of this is a reaction to certain large cities that derisively refer to the rest of the US as "fly-over" country, but part of it is simply that Arizona has some of the youngest sports franchises and in the country. Legitimacy has many paths, but sometimes it feels like Arizona hopes that posing as a proper franchise and fanbase, it will become one.
As such Arizona sometimes is quick to declare rivals, because its big brothers all have rivals. But the Brewers are not our rivals. Booing Prince Fielder was a kind of fun, one time thing, but it was a product of a particular time. It’s not going to carry over into the regular season, any more than the 2007 NLCS created a long lasting rivalry with the Rockies. At least with the latter we can make a better case, but the truth is that any rivalry wasn’t sustained.
Do we even need rivals? Most of them are rather one sided. Lakers fans really don’t think about the Suns; it’s a hatred that seemingly flows only one way. Even worse, often times the participants define themselves by who they hate. There’s something pathetic about people who would rather celebrate their hatred of another team, instead the love of their actual team.
Rivalries won’t boost attendance at Chase. If the team is winning, people will go (with some delay).