I don't have sources that outline the issues on why taking it for a long time is bad. I have talked to a pharmacist and other articles stating that it's a temporary fix.
Here's another article mentioning that it should be taken for 3 months at most:
The three months is because there have been studies showing it is safe for that long. But afaik there have been no studies suggesting that it would not be safe to take it for longer.
Here's another article mentioning that it should be taken for 3 months at most:
http://www.huffingtonpost.com/dr-michael-j-breus/melatonin-s...