Facebook filters your newsfeed according to your activity. The more you click on, like, and comment on posts by a particular person or page, the more you will see post from that person or page at the exclusion of other people and pages. The more you click on, like, and comment on a certain type of post (politics, cats, liberal, conservative etc.) the more you see posts of these types at the exclusion of others. This results in a Facebook newsfeed that is filled only with things you like and agree with, and has very little posts with viewpoints different than your own.
Photo credit: Sean MacEntee