Harry and Louis were just normal best friends. Maybe not normal since they're in the world's biggest boy band, but close to normal.
Their management wanted fame and money so they did everything in their power to get it. They forced Louis and Harry to pretend that they were in love and needed to hide it from the rest of the world because of modest who thought it would give them bad publicity.
Millions of fans hated modest but at least they had a story, they got money and people new who they were. The fans talked about them all the time.
Some fans really believed that Harry and Louis were in love and they called themselves larries. The rest of the fandom hated them.
The two young boys then fell in love.