A lender is not interested in being a homeowner, which is why they are lending the money to someone else who is. Clearly, the borrower failing to live up to their obligation to pay the mortgage as agreed has a negative impact on the lender. The lender agreed to provide a loan to the borrower. The lender did not agree to assume the risk of the value of the home declining. That risk is borne by the borrower, just like the borrower reaps the reward of an increase in value on that same property.
Why didn't you answer the questions I posed?
If someone has a home, lives in it and is paying the mortgage, how does being "underwater" impact their ability to pay said mortgage?