How and why are the media and politicians dependent on each other?

Media and politicians are dependent on each other, because of how they can change a person's view. 

Media, whether it be a talk show, advertisement, etc, can be used to give people 'positive' or 'negative' "opinions" at large, while putting in some facts to make the whole look like it's completely true. 

To be able to get people to vote for the politician they want, there are a couple way they can do this.

Bias: Whether positively or negatively, some politicians may get longer 'air time', as well as people giving their opinions (presented as facts) to help swing votes. For example, people can say that "Obama is the best president ever because of such~and~such things he do" (positive bias), OR they can say "He is the worst president because he allowed America to fall from her great power (negative bias)
Polls: Whether it is true or not, polls CAN affect how people vote. Some people vote purely to be "within the crowds", and, when seeing large amounts of people vote for a certain group, politician, or party, they will also vote for them. However, this does not affect everyone. Media can skew charts to make it different from reality, whether positively or negatively, to change how people vote (as some follow bandwagon)

In exchange, Politicians usually help the media, whether provide funds, or (if they are elected) give favors to the media.

In the end, It is for money, familiarization, popularity, or anything other, politicians and media will always work together to create 'pictures' for the public to digest before and when voting.

hope this helps