I should have qualified that by saying 'Top Goalkeepers'. Obviously you can't compare the likes of Scott Carson to a trained figther (well maybe to David Price..), but I think top keepers share some characteristics. They need to be brave, tough and incredibly confident as they face the prospect of humiliation every time they perform. A moments doubt or hesitation can cost them completely and when it does they need the mental strength and self belief to bounce back and still believe they are unbeatable. In terms of mentality, I am referring to how they view themselves - with unshakeable self belief.
It's not a question of whether boxers are tougher, of course they are, but the goalkeeper is the loneliest and most pressured position on the pitch and to deal with that 'keepers often adopt a similar mentality. You wouldn't want your fighter stepping into the ring thinking they were going to lose, just as you wouldn't want your keeper going onto the pitch thinking they were going to concede!