The Nintendo Headquarters saw Luigi as equal to Mario… he had the same design only with swapped colors (which look way cooler than his new ones, in my opinion!). They’re the same, they’re men of the same worth.
That’s until they chose to have him go second, and name him Luigi after “Ruigi” which in Japanese means “similar”. Then they swapped his colors, put him in second place, changed his design over the years to show us that he’s not number one. Even his victory cry after a race won in the Mario Kart series is “who’s number one now!”. This points to his feeling of insecurity, which is also explored in the “Mario & Luigi” games. He got the personality of someone insecure, the second best. But then again, when fooled into believing he’s Mario in the Mario & Luigi series, he’s physically got the same potential and is able to achieve the same things. Basically we have in Luigi a man stunted by his bad self esteem.
And that explains why they created “Year of Luigi” in 2013. They knew what they’d done. They’d stripped a man out of his confidence and feeling of self-worth. By doing this big thing I think they gave some of it back to Luigi, restoring him to his former grandeur as the one who took Mario’s place when he’d fail, that same man on that lunch box from the past. Just look at that confident face. Who couldn’t trust that guy to save Mushroom Kingdom?