This stems from people not understanding the difference between a republican and democrat.
Cracked.com had a good article on this i wish i could find.
Ultimately, the only difference is that one believes in big government and 1 believes in small government. Thats where the term blue dog democrat and rhino republicans come from. Theres ideas such as abortion and gay marriage that we traditionally say 1 party supports and the other doesnt. These arent actually party issues though. This most recent election, for example was talked about as a liberal vs a conservative more than a republican vs democrat. Honestly the term republican and democrat is largly misleading. There are republicans that agree with gay marriage just like how there are democrats that believe abortion is wrong. If im remembering correctly (i could be wrong) I think in the vice presidential debate, Joe Biden said while he personally disagrees with abortion, its not a belief he would force on someone.
The whole party system as a whole needs to be demolished in my humble opinion. There was a lot of talk about Chris Christy joining the presidential primary, but he has a lot of views that dont go with the republican party. This system is nice because it helps the uninformed voters, but this system is bad because it helps the uninformed voters.
Its bad because people will see "R" or "D" and vote for the person, without knowing about what s/he believes. This also causes the divide in the population like what we have today. Republicans believe that all democrats are big government, gay loving, pro abortion, big debt fools, and democrats think republicans are all abortion hating, small government, gay hating, anti abortion, stingy fools. (note I know, no one is pro abortion, they are pro choice, im making a point here).