What is unjust or inefficient about your example with purchasing the house?
Should Dave not be allowed to borrow money to own a house? Should only the wealthy, with sufficient cash reserves, be permitted to buy a house, without financing?
If you don't value the house as much as Dave -- on a net present value of the future payments you have to make to own it -- why should you have it, instead of Dave?
Should real estate prices be capped, by a central planning board, so that Dave cannot borrow too much to own it? If so how does the seller choose who to sell to, between you and Dave? Who says the house is worth X, and not Y, when Dave is happy to pay Y?
Or is it unjust that Dave likes the house enough to commit much of his future income to it?
Why is Dave a moron? Because he disagrees with you on what to use his future income for? Should you be able to tell Dave what to spend money on, and what not, out of your view of what is socially efficient?
Perhaps not just you; you could get a few of your neighbors together and call a vote on what Dave can spend his money on. Based on social utility, of course.
But to go back to your example. Suppose you choose not to outbid Dave, because you are smarter, and instead opt to rent an efficiency studio and save up over your life and build a sizable amount of investment assets (because you did not spend frivolously on a large house). As you age, you look for your savings to work for you, because you were smart enough to defer consumption and now you feel you should be rewarded.
Where to put your assets? There are many choices. Should one of them not be the option to lend Dave's son, Dave Jr., money for his own house, in return for a level of interest to compensate you for use of your capital? But that's a pain to find the borrower and vet him, and risky if it's just relying on Dave Jr (who may be a moron like his father).
What if there was an agent you could pay, who would originate the loan, check that the title was valid, check Dave Jr.'s credit, author the loan documents, and then provide it to investors like you -- or even bundle them together, so that you could spread risk of default over a few different mortgages, instead of just one?
That would be great -- you could earn a return on your saved capital, and Dave Jr. would get what he wants too (even if you disagree with what he should want)!
Ah, but of course, agents like that are just parasites. So instead of reinvesting your saved capital with someone willing to borrow the money, you stuff it in your mattress.
To make society more efficient.