A person who is optimistic and easy-going; someone who tends not to worry and looks on the bright side.
彼は生来の楽天家で、小さな失敗ですぐに立ち直る。
He's a born optimist and bounces back quickly from small failures.
親戚の中には楽天家もいれば、とても慎重な人もいる。
Among my relatives there are optimists and also very cautious people.
楽天家だからと言って準備を怠っていいわけではない。
Being optimistic is no excuse for neglecting preparation.