It's a little bit of everything.
It's easier to be brave when you're clueless.
Joe
Is very true - when you don't know something you don't have any idea of what it is that you don't know - so yes its very easy to be bold and confident and make a total pigs ear of it.
Then there is the once-off nature of the shoot. Whilst most if nearly all non-controlled situation photography is a once off affair; most of the time if you miss the shot its not a big deal. Sure you might not get paid if you miss the shot; but that's about it for most situations - or the shot will come around again some time (or one similar). With weddings its 100% one-time-only coupled with a lot of emotional attachment and often money (relative to the persons income - a £50 is generally £50 cause hte B&G don't have more to pay). This makes it very intimidating - its a one time affair - you've GOT to get the shot.
Then there is the nature of a wedding itself - they vary lots, but overall they are chaotic events for a photographer. Because you're not the centre of attention - there are loads of things going on that are not your area. So you've got to organise others - get them ready and get the shot fast and move on and get the next shot.
Then you've got the "Ohh its a wedding scary" vibe that is repeated all over the place which reinforces the fear factor
The only way to overcome most of those problems is to practice - ideally by second shooting. Practice will overcome many but it won't overcome the biggest.
Desire. If you desire and really want to shoot weddings you will - might be as a free intern/second shooter or as the primary. If you want to do it you generally can. If you don't then chances are you won't - or if you do you'll run the risk of doing a bad job of it.