World War I
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, claiming to be a tomato fairy and was frightened of Germany. Germany had doubts if this was really Roman's ancestor and took him as a prisoner, but kicked him out after he became too annoying to handle.
Even so, Italy managed to come back and Germany kicked him out again, repeating this action for a few times before completely letting Italy stay at his house. Germany couldn't believe the degree of Italy's willingness to stay as his prisoner, leading him to gesture at the door and shouting that it's not locked. Italy stepped slowly out of the house and he chatted happily with the girls before going back into the house while Germany could only stare in disbelief.
During his stay, Italy eventually got bored and created a song for Germany.
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.
World War II
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, especially ones where he got captured by the Allies. Most of these situations were started with Italy phoning Germany, screaming for help. He responded by sending letters of Italy's preferences to the Allies, the other side also found Italy's whining a tad too much and strapped him into a box to get send to Germany.
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.
Buon San Valentino
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 third 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 Feliciano, 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.