Every reporter has an opinion and it is very hard for them to not let that opinion creep into their journalism. The same way that a newspaper editor may want to be fair but will pick articles that either will sell more papers or that he or she agrees with more.
We can get information and opinions with just a push of a few buttons, It is not hard to form your own opinions anymore. There is just to much information. We can't really say, "I just do not have enough information to form an opinion." The media is also drawn into this behavior. Many times, they probably do not even know they are doing it.
Personally, I do believe that the media is biased, but I also think that it's unintentional. Reporters likely have their own opinions on issues and, for example, their favorite Presidential candidates. I most feel like I see this on election night for the President. You can tell which reporters are getting excited when their guy is winning. So even though I believe the media is biased, I think it's because reporters are human and have their own views on issues, which inadvertently biases them.
I do think that most media bias is intentional. It is clear that some networks and channels have a liberal bias. I think most of them try to report their news that favors the Democrats and liberals. This is all done intentionally. While there is bias across all politics, the liberal bias is the most apparent.
The bottom line is every media story is written by someone. No matter how hard you try to be neutral, the human brain automatically makes some judgments. There is no way to write about a subject unless you know something about it, and once you know about it, you form an opinion. Media bias is based on the medias opinion.