Having gone thru the sec clearance thing in the 90s, the third thing is if you have financial issues (like an expensive addiction with much income) and some foreign intelligence service can "help". So they're pretty interested in finances. Which wouldn't have been a problem for Jobs...
A friend in college wanted to be an FBI agent, so I got to hear alot about this.
I believed they polygraphed you about drug use, and I recall that they had a threshold number of "experimental" sessions with marijuana that were ok, as long as you disclosed them during the background check and polygraph.
1. Whether you're likely to voluntarily leak any secure information.
2. Whether someone who dug up some dirt on you could blackmail you into leaking secure information.
Or as the saying goes, it's fine to have a mistress, but having a mistress that your wife doesn't know about is a problem...