A story about Tom and Bill..........

A few years ago Tom asked Bill for 500 dollars. Bill had plenty of money in his account so he gave Bill the cash. Six months later he asked Bill for 500 more. Bill knew he was going through hard times, so he gave him 500 more. A few months later Bill gave him 500 more.
A few months later Tom asked Bill for 500 more. Bill told him no because he also was going through rough times. Tom told Bill he is a shitty friend. What lesson could Bill have learned from all this ??