Don't Shout About What You Have!
Here is a hand which demonstrates why you have to think carefully before making the "obvious" lead.
My partner and I alight in the standard contract of 4H and North leads his singleton diamond. Since we have bid and agreed this suit, either North is someone with the IQ of a certain US President, or this is a singleton. Since North is known to be quite a decent player, everyone knew it was a singleton: the declarer; South; dummy; people at the next table, the neighbours; even the fox rooting around in the skip in the next road along.
The problem with the lead is that it picked up South's Qd and alerted declarer to the need to drew trumps immediately. As a result, 4H made comfortably.
At other tables, where North had not led his diamond, most declarers played for Qd to drop in two rounds and, as a result, lost two spades, Ac and Qd.
What makes the singleton lead so poor here is that North himself holds the entries that he really wants his partner to hold. If North has not held either black-suit ace, then the lead would have been worth the risk. As it is, all it did was shout about the singleton and expose partner to a first round finesse in a suit that had been bid and agreed by the opponents.
Only lead a doubleton if you are very weak indeed, or if partner has called the suit. Never lead a doubleton in a suit the opponents have bid.
Only lead a singleton when you feel there is a strong chance that partner will be able to provide a ruff for you.
(Apologies to my very good Cape Town friend, Sean - North - but I've spent my life being told off at the bridge table and, maybe - just maybe - these days, I finally have some idea what I'm doing...)