For a start, a 'brain' is different from a geek or a nerd. Not too different, but just enough. Basically, a brain is a pretty normal yet very intelligent person (in other words, a member of E2).
Brains make the best friends because they're always there for you. They help out in times of trouble, and are a shoulder to cry on when times get tough.
Brains have a better social code than most people, and usually have higher morals. This means that they don't backstab nearly as much, and are very loyal to friends. They also don't create rumours.
When a Brain argues with another friend Brain, the arguments are constructive, and less prone to violence than a normal argument. Brains listen to each others opinions, and work things out by finding an alternative to the problem or a middle ground.
Brain guys make great boyfriends or partners because they are able to hold a conversation, and an intelligent one at that.
Brain girls make great girlfriends or partners because they are able to put up with silences (because the guy will be thinking something important), and understand the need to not have somebody nagging the whole time.
Contrary to popular belief, Brains can also have fun. Brains enjoy going out, and 'hanging out' with friends. Brains can be fun to hang around.
Conclusion: Brainy people make the best friends!