Borrow/Lend = Return/Not Return?

To Borrow or lend implies that the commodity is supposed to be returned, right? So when people borrow stuff, especially money(irrespective of the amount - big or small) why do they conviniently forget to return, more often than not?

As a principle I beleive in returning every pie I borrow, if at all I do ever happen to borrow. I try avoid such transactions, unless inevitable and hence expect the same incase I lend. I believe money can mar a relationship irrespective of how close you are with the other person. It's not about how much(I have had people telling me "it is such a small amount, why do you care?") but about the principle.

Nature only knows whether such people have a conscience!