The general perception seems to be that democrats are in favor of the rights of the working class, women, African Americans, and homosexuals while the republicans want to hold minorities back and mistreat women and homosexuals.
Part of the problem is that the two main parties in this country are too polarized, and often their stances simply don't make sense. The democrats want to tighten gun controls (because, after all, criminals are going to obey the law!) and republicans want to ban abortion (because there was no abortion at all before Rowe v. Wade). Democrats want to tax the wealthy to give to the poor (taking the rights from one individual in order to give rights to another) and republicans want to ban gay marriage (taking the rights from one individual in order to give the rights to another).
In so many ways, the two parties are no different from one another; they only have chosen different issues and have taken polarized stances on those issues instead of paying any attention to what the constitution *says* about the issues that come up time and time again. In other words, both parties are "playing politics."
Perceptions aren't always correct. Margaret Sanger hoped that African Americans would abort themselves out of existence if abortion clinics were provided in the proper neighborhoods, the KKK was built on the backs of anti-republican democrats and MLK Jr. was a republican (as are his heirs). People listen to propaganda and it's very difficult to change minds once a person has it ingrained into their view of history. I've known people to reject the very words of the founding fathers when they don't support their viewpoint on gun control, speech control or the tenth amendment.
But the answer can be summed up in a few short words: People view the democrats as the "good guys" because the democrats give them "stuff" and they (the dems) give them stuff for nothing that previously belonged to someone else who had to work for it.
After all, Robin Hood was the good guy, and the Sheriff was the bad guy. Right?