I’ll give you an example. The money you have is $1 and let's look into the next two options. And let's suppose house prices go up by 10 percent overall.
1. Buy a $1 house => Rise to $1.10 => Earn 10 cents from a $1 investment => Yield 10%.
2. Get a $9 loan and buy a $10 house with my $1 => The house price goes up to $11 => Pay back the $9 loan and the remaining $2 => Invest in $1 and earn $1 => Yield 100%.
(Of course, you'll have to consider the interest rate because you'll have to pay the interest on the loan. For an easy example, interest rates are not considered).
This is called the leverage effect. So the appropriate debt ratio is a positive view.