As Southwest explains it (Aviation Daily subscriber-only story), its decision to add Denver as its second route out of Minneapolis-St. Paul on May 26 makes sense on several levels: not only because of the origin-and-destination market--and because Denver has been the fastest-growing newly served city in Southwest's history--but also because of the connections customers can make via Denver to the West, West Coast and Southwest via Denver.
This is the same reasoning (Aviation Daily subscriber-only story) that was behind Southwest's decision to make Chicago Midway its first MSP market, with the eight daily flights having begun March 9. Southwest is such a big presence at Midway that passengers departing from MSP can connect via Midway to 57 other destinations. But take a look at the Southwest route maps below for its service out of Midway and Denver, and in particular at the western and southwestern destinations served.
What you may notice is that Southwest serves about a dozen of the same destinations in the West and Southwest out of both Chicago and Denver. This means, in a sense, that Southwest will be competing against itself for connecting traffic.
Of course, Southwest can make a legitimate case that customers would prefer to connect out of Denver regardless--since that's a more direct route than going to Chicago first--and that customers already have other airlines they can take from MSP to Denver and beyond. That's especially true for United and Frontier, which have Denver hubs and which offer five and four daily flights, respectively, between MSP and Denver. All in all, with Southwest also challenging Northwest out of its MSP hub (from which it offers six daily flights to Denver), this is shaping up as one heck of a competition to watch.