Others have made suggestions why, but it's pretty clear in his answer regarding Wikileaks, if you read his between the lines.
He believes that he is only disclosing things he believes are in the public interest. He is specifically withholding any information about "US operations against legitimate military targets".
Wikileaks dumped a bunch of unredacted information because someone screwed up. That wasn't their intent (the group, not necessarily Assange). Snowden appears to want to prevent this from occurring again.
<...”He believes that he is only disclosing things he believes are in the public interest”....>
That’s a lie...and he knows it.....what he’s really saying is he’s in charge....and will use stolen information at his discretion when and how he wants.
No secret he went after the UK Summit after they refused him to board any of their airlines.....
No secret he revealed information of spying on China when O and the Chinese leader were meeting the first time.
He’s playing cards with the big boys and this has nothing to do with the American people. He’s clearly in over his head..but has yet to realize that...fool says it well.