The two are traveling together out of convenience. They were a pitcher and catcher on a minor league ball team, and are the only ones who survived whatever happened.
After Mickey hears two people talking via the walkie talkies he found, he tries to make contact. But they are not interested and tell him to get off their channel, don't contact them again, and don't try to locate them. Since they don't want to be found, Ben wants to leave item alone, but Mickey longs for companionship and keeps trying to contact Annie, the girl they overheard. In the aftermath of an apocalypse, it's probably best to stay away from people who tell you they don't want to be found.
If you go into this film expecting a lot of zombie action, you'll be sorely disappointed. It's about Ben and Mickey's interactions, and the difference in personality. These are two people who wouldn't have been friends, but are forced together due to circumstances. Mickey's a nice guy, but unable to deal with this situation and spends his time escaping into music. He leaves killing zombies and scavenging to Mickey, who is a jerk, but more survival oriented than Mickey.
There are long scenes with no dialogue, and the scenes with dialogue are mostly a character study. Ben has no problem killing, and even forces Mickey to kill by locking him a room with a zombie. But Mickey's sensitivity puts them in danger because he is unable to make decisions based on survival, rather than need for human contact.
This movie isn't for everyone. The film is a bit longer than I would have liked, clocking in at one hundred minutes, and I must admit there were some times where I fast forwarded in the movie. So if you are looking for a different type of zombie movie, and are okay with movies based on dialogue rather than action, then maybe you'll like this. I didn't love it, but didn't hate it either.