Let me use a reductio ad absurdum argument for a minute. (A special case of which is proof by contradiction.) Wikipedia summarizes reductio ad absurdum as
a form of argument which attempts either to disprove a statement by showing it inevitably leads to a ridiculous, absurd, or impractical conclusion, or to prove one by showing that if it were not true, the result would be absurd or impossible.
With the preliminaries out of the way, let's dig in...
The intention of having a debt to income ratio limit in the first place is to ensure that debtors don't take on more debt than the lender can expect the debtor to be able to service. (To service a debt is to make interest and capital payments on the loan amount according to the payment schedule of the loan.)
Suppose that the debt to income ratio that a lender would look at does not include the loan under consideration, but rather only your previous debts.
In that case, a person who has zero debt outstanding to begin with could borrow any amount, because as long as they have a non-zero income, their debt to income ratio would be zero.
Since there exists some upper limit on how much debt a person can service given any particular level of income, this means that going by debt to income ratio alone, by excluding the loan under consideration, a person could easily borrow far more than they are able to make good on.
This is clearly not reasonable from the point of view of the lender, who presumably want you to be able to make both interest and capital payments.
Consequently, the debt to income ratio as considered by the lender must include the loan under consideration.
