Sometimes a couple will qualify for a mortgage with one partner’s income, but won’t qualify with their combined income and debt. Think new lawyer with $200,000 in student loans. Think recently divorced and remarried person still carrying the bad credit from his/her ex.
You would think that lenders wouldn’t allow only one person to be on the mortgage and two on the Deed. But they do. Why?
Legally married couples are responsible for their spouse’s necessary debts, whether they signed up for them or not. Mortgages are necessary debts. Therefore, lenders are not concerned that they have allowed the spouse of their borrower to be on the house Deed. They can still collect from that spouse. 
This will not be the case for unmarried couples. In that case, the partner cannot be on the Deed unless he or she is also on the mortgage. If the couple subsequently marries, the deed can be amended.
(Source: Mike Krone, Kriss Law)
