Why the hell are people saying Republicans will gain seats in the senate?

[Southern Senator North Carolina Yankee]

Maybe because Republicans have double digit leads in three Democratic held seats (WV, SD, and MT), are tied in two more (LA and NC), and trailing in still two more by three or less (AR and CO) with demographics working at least somewhat in favor of the Republicans in one and somewhat in favor of the Democrats in the other. That still leaves yet one more double digit Romney state with a Democratic incumbent and a solid GOP challenger who is running away with the primary (Alaska) and a swing state that for the all the hype of the GOP dropping the ball, the Democrats have yet to fully secure (Iowa).

The Democrats have a chance at just two GOP seats, Kentucky and Georgia. In former you have a long term incumbent who is facing voter fatigue and in Georgia you have a severely divided GOP primary field that is hampering all the candidates in matchups against hand picked singular Democratic candidate. In both states, Democrats have struggled considerably in recent years and whilst the numbers indicate that is beginning to change in one, it is hard to label either as anything less than hostile territory for the Democrats. 

To look at it more broadly, the reason is that the GOP has a good map (more Dems then Reps) yet one that is heavily skewed towards Southern and Western states (courtesy of 2008) in the second midterm of a Democratic President. The GOP could have done much better in NC, NH and IA with regards to candidates, but that doesn't change the significant advantages the GOP has right now in terms of just making gains. The question that may be determined by those mistakes and any others to come is whether they are able to gain control of the chamber. Secondarily, if they gain control, whether it is by enough to whether the storm in 2016 and get to a second shot at class 1 in 2018 where they can reasonably target at least four seats in GOP territory (IN, MO, ND and MT) and two or three more in swing territory, with just eight seats to defend mostly in deep GOP territory (UT, WY, NE, MS, TX, TN, leaving only NV and maybe AZ at risk).