Sunny means shining with bright sunlight, like a beautiful sunny Saturday at the beach. It can also mean cheerful, like your sunny, upbeat best friend.
When the sun is shining, it's a sunny day, and when you're smiling and friendly, you're sunny too. Sunny has been used in this figurative way since the 16th century, and in the 1830s, sunny side came to mean "optimistic outlook." However, if you order your eggs sunny side up, it just means you'll get two fried eggs with bright yellow, runny yolks.