I've also given away lage amounts of money - not as loans - over the years without the expectation of being repaid, so it's not a question of generosity.
Simply put, it is not OK to promise someone to payback within X time and not do it. If you are not sure you would be able to pay back, make it clear upfront. If something out of your control happens, it's OK. I agree with you that in that case we should be nice to each other and help each other.
Also I'm not considering interest rates here, just paying back the loan as is.