By Democrats I mean left-leaning people and by Republicans I mean right-leaning people. I used those terms in the title for convenience, not precision, and I intend for this question to be open to all users, not only Americans. I also want to make clear that I'm asking about a general trend, and not insinuating that all or most leftists do this.
With all that being said, I want to find out why this tends to happen unidirectionally.