There are monetary debts, and there are tangible debts such as when someone owes someone else a favour. Both these are easy to identify. But then there are emotional debts which are not easy to identify. Maybe it is wrong to refer to them as debts because the term is transactional. But I use it for lack of a better term and also because some of us do keep a mental tally of these things.
I recently helped someone out, and although I would never remind them of it or call in my debt, it was recorded in my mental balance sheet. I then realized that that person had come into my life immediately after a traumatic incident in my life. That person was a companion to me during a difficult period and it is only later that I realized what a difference their presence made during that difficult time. They did not counsel me about my loss and I barely spoke about it to them. But just their presence was a great source of comfort, as I later realized. And that realization changed my perspective on our debt relationship. I was the one who is in their debt, not they.