Because the implication is that there is non-monetary profit. The person is describing it not as a loss but as an investment, because it was an investment in themselves and their ability to sleep at night and an investment in their child feeling safe and whatever other impossible-to-directly-price externalities were realized as a result of the monetary loss.
I suppose the problem is that there isn't a good word for this concept. I just find it interesting that, in my experience, this is only used in relation to luxury goods.
> in my experience, this is only used in relation to luxury goods.
Not really. You can also "invest" in personal relationships, for example, which implies neither a financial outlay nor an expectation of financial return.
When used in the context of purchasing consumer goods, I think it is most often used in the context of goods which are expensive (the financial outlay) and durable (ie, capable of providing enjoyment--analogous to profit--for a long time into the future). And of course those goods are more likely to be considered "luxury" goods.
In the case of a relationship, maybe it could be argued that you are profiting, in the currency being used: you get more positive interactions out than you put in. ;)
