Thomas Zoth (note spoilers disclaimer):

And Joe? Always so cool with his smoking and drinking and apparent disinterest in his attractive young co-workers? He’s actually suicidal. His self-destructive impulses were, somewhat surprisingly, self-destructive.

My view of Joe is less grim and dark than this one. What I think is that Joe is living in a dream.

In real life Joe is a Toudai graduate who has somehow wound up in what is basically a relatively unimportant and certainly unimpressive civil service job. Outside of the office he has a second life as the classical heroic badass, too cool for words to contain; he hangs out drinking in bars looking suave and acting mysterious, for example. One way to interpret this based on the information the show has given us so far is to view Joe as Walter Mitty with actual powers. His oh so cool life as a Gatchaman is fundamentally an illusion and an act, an escape from the mundanity of his day to day drudge and something that gives his life importance and meaning. He acts the way he does because this is how heroic badasses are supposed to act; he is deliberately living out the cliche whether he admits it to himself or not. His entire self is defined and sustained by being the hero, not the civil servant.

(Spoiler warning.)

In this reading, Joe collapses against Berg-Katze in episode 7 because Berg-Katze systematically destroys Joe's dream life by shattering the illusion. There is a really revealing dialog at the end of the fight; after Joe has been unable to do anything to Berg-Katze, Berg-Katze plays the voice of Joe's inner self and digs in to Joe with the following:

[...] I can't possibly win. It's impossible. I acted tough until now, but in the end, I'm just a civil servant. I wanted to make it big, but I knew my limitations from the beginning, really. Oh, well. I guess I'll just keep trailing on in life. Not like there's a whole lot I can do, anyway. I should just give up on my dreams and resign myself to being a nobody. [...]

This is Joe's secret and soul-destroying terror: that his daytime life is the reality and his nighttime cool heroism is the illusion. Forced to face it, Joe lacks the strength to go on anyways and his doubts consume him.

(The translation I'm using comes from Commie's fansubs.)