I hear people saying, in looking back at his career, how honorable General Petraeus is — but that is not exactly right. In fact, that is wrong. He is a cheater (don’t feel sorry for him – it was his choice) and that is not exactly honorable.
From the nation’s perspective, he was a General (how long has this affair been going on?) and the Head of the CIA. Both those jobs are very powerful and very vulnerable to blackmail from those with very bad intentions against the United States. How long has this been going on?
And why did he suddenly go public? My guess (only a guess) - he got caught.