The House star said it was because of his ancestors north of the border.
He told Scots-born talk show host Craig Ferguson, 'I am very pessimistic.
'I hope you won't mind but I actually attribute this to my Scottish roots.
'Dour.'
Laurie, 51, added, 'I definitely think that's where it's from.'
Ferguson said, 'Scottish people are terribly pessimistic.
'The basic feeling that I got whenever I tried to do anything while I was in Scotland I always thought people were thinking, 'You think you're great but you're not'.'
'These people would be my family,' he joked. 'I think the English are quite pessimistic too.'
Oxford-born Laurie has Scots parents, according to reports.