This is problematic because reality TV, of course, is not exactly just real life on camera. Instead, the shows are edited to make them more interesting and exciting. What this tends to mean is that they get edited to include more conflict, more danger, more of things that you could call negative. When viewers watch this sort of show, they believe that reality is like that -- that reality is typically full of conflict and other negative events.
Therefore, when people watch reality TV, they can end up having a more negative perception of real life than is actually warranted.
Another significant negative impact of watching reality television is that viewers allow someone else to determine "reality" for them. Orwell depicts this danger in 1984 in which humans have begun to believe everything that is depicted on the large telescreens. While 1984 is fictional and was written over 60 years ago, it portrays humanity's seemingly natural tendency to be easily swayed by what they "see." History has shown that when a mass of people can be easily controlled by a single person or a small group, the results are disastrous.
Secondly, reality television and those who produce it should cause us to consider the ethics of the process. Is it ethical to exploit someone's divorce, incarceration, and teenage pregnancy for entertainment's sake? Some justify the airing of these events in humans' lives by claiming that the subjects of the shows agree to be filmed and that they are compensated for their "stories," but what does is it say about the producers and viewers who are willing to make a profit or be entertained by someone else's pain, immaturity, or poor choices?
For excellent satire of the phenomenon of reality TV's popularity, you should watch the film The Truman...