In a short period of time, stealing secrets has gone from the laborious task of copying papers taken surreptitiously from filing cabinets to the current age in which files can be electronically copied onto thumb drives. Manning was said to have disguised his efforts by downloading secrets onto compact discs made to look like pop music recordings.

“Bradley Manning could have extracted documents in the past, maybe even a briefcase full,” said Joseph S. Nye, Jr., a Harvard Kennedy School professor and author of the book “Presidential Leadership and the Creation of the American Era.” “But the idea that you can get a warehouse full in a Lady Gaga disc — that’s new.”

As for Snowden, his access to the top secret data might have made it technically impossible to stop him, analysts said.

Money, ideology (patriotism, religion), coercion, ego/self-importance, excitement, disaffection/grudges, personal relations, sex--it's all right there in the manual. 

Manning was an idiot.

Snowden, however, might have a point.  His motivations might be ideological, but for several red flags.  His "girlfriend" articles about pole-dancing and other posts make it sound like she was fictional or a recruiter, so coercion or sex.  Also his health issues make him sound like he might have had a grudge as that would put him at risk at being removed from his job.  The articles about him come across as earnest, so I would discount the Assange (ego) factor.  

The single biggest factor though is his supposed fiancee said they were going to be married, but now she is rudderless. So...runaway groom?  I think we can sell it to Cosmo.