During World War I, Germany went to search for the heir of the Roman Empire, wishing to fight him. He came across a wooden crate in the middle of the woods, and opened it to find a panicking Italy inside. Germany took him as a prisoner, but kicked him out after he became too annoying to handle.
Following the war, Italy nearly starved to death in the webcomic version, while in the manga and anime, he became poor due to the fallout from the war. Either way, he had to go back to work for Germany.
When World War II started, Italy wanted to become Germany's ally. While refusing at first, Germany eventually accepted the alliance, and the two agreed to help each other. Germany soon found the alliance frustrating since he had to constantly bail Italy out of situations. In addition, Italy often ran away from battle, flirting with girls instead.
Italy's older brother Romano hated Germany. When Germany became annoyed by Italy's friendship posters, Romano tried to convince Italy that Germany hated him. Because of this, Italy ran through Europe naked to verify this claim was false. Germany, who was also naked, blushed and said he didn't hate Italy. Italy demanded more clarification, to which Germany responded that he "loved" Italy.
One Valentine's Day, Germany bought Italy a bouquet of red roses, a German confession of love. Italy was unaware the gesture was romantic, while Germany was starting to believe Italy had feelings for him and not any of the girls Italy had flirted with. Italy told Germany he had a first love, and that it was a boy, causing confusion.
Gerita is one of the most popular ships in the fandom. On AO3, it's is the fourth most written ship within the Hetalia: Axis Powers fandom tag; Germany's most written, as well as North Italy's most written.
It is speculated that Holy Roman Empire, the childhood love of Italy, became Germany. Some evidence for this is that the Holy Roman Empire was located roughly where Germany is now, as well as the physical similarities between the two and the lack of official pictures of younger Germany. It is speculated that after the disappearance of the Holy Roman Empire, he lost his memory (or purposefully blocked memories out) and became Germany.