I don't agree with it first of all on feminist grounds. "Be a man" is one of the most sexist statements I hear in common use. Not only does it imply that being a non-man, ie. female, is something that makes one emotional and unstable, it also shoehorns all men everywhere into an emotional iron maiden. The idea that "a man" is stoic and relatively emotionless causes all kinds of untold damage due to repressed issues that need to be dealt with. Let me take rural Australia for example. There's been a drought here for the past decade or so. Out in rural areas that takes its toll. Men who ascribe to the Stoic and Manly ideal find it difficult to seek help for the problems, such as depression, that arise out of having a failing farm when it's been in the family for generations. Stereotype, but you get the idea. The suicide rate in those areas and in this situation shoots right up.
The other reason it irks me is because it demonstrates a fundamental misunderstanding about mental illness. I differentiate here between medical depression and "I'm down, need someone to talk to." While the latter is valid as a request, the former runs far deeper. For someone with depression it is impossible to "be a man" and cheer up because of chemical imbalances and other physiological factors upon which no amount of stoicism can have an effect. Stephen Fry related an anecdote during an interview about a fellow bi-polar man in the depressive swing of his illness. This guy had walked in front of a truck and wound up spending six months having the bones in his legs re-broken and re-set repeatedly in an effort to enable him to walk again. The guy said that while yes, the pain of having his legs mended was truly terrible, "it was nothing compared to the pain that made me step onto the highway."
Manic depressives do have the manic part of that cycle to reassure them. It will get better for them because that's a part of their illness. People with depression don't, and it's not fair to burden them with masculinity crap atop everything else. It won't make them better, it could well make them worse. The most likely result is that they just stop talking about their troubles.