The pleasant or cheerful aspect of something, as in Beth always sees the sunny side of events like graduations. This idiom alludes to the area on which sunlight falls.
[ First half of 1800s
on the sunny side of. At an age less than, younger than, as in He's still on the sunny side of forty.
[ Second half of 1800s
sunny-side up. Fried so that the yolk remains intact and uppermost, as in I ordered my eggs sunny-side up and you brought me scrambled eggs. This expression transfers the appearance of the sun to that of an egg yolk.
[ c. 1900