As I understand, when trying to sum the variances of two correlated random variables, the formula is such:
$Var(X+Y) = Var(X) + Var(Y) + 2Cov(X, Y)$
My question is, why is the last term multiplied by two? I feel like I am just missing something really obvious, but I can't wrap my head around why it's necessary. Maybe someone can explain for me?