Here are some possibilities:
* He may have a fear that he keeps private. It's common for people to keep personal fears to themselves.
* He might be comfortable with his fears and not feel the need to share them. Some people embrace their fears and work through them without making a big deal about it.
If you're interested in learning more about Jensen Ackles, you can check out his social media accounts or interviews he's given. He might mention a fear casually at some point, but it's not something he's publicly discussed.