star wars - Why do the Jedi insist on the concept of "No Attachment"?

At the surface, it sounds like a complete butchering of the Buddhist concept of attachment, which is not what most people think, and has more to do with more to do with denying ourselves suffering and pain (the dukkha) than forcing ourselves to become closer to fellow human beings. The tenet is most often expressed as the fact the Jedi don't marry - which is closer aligned to Catholicism with roots in theology.

Considering:

1. There isn't anything central to the "Light Side" which prohibits love, romance or marriage


2. The Jedi's restrictions on Anakins emotions ...

... help create the greatest mass murderer that the Galaxy has ever seen

Is there some in-unviverse basis as to why this concept is even remotely a good idea? This is noting that the New Jedi Order seems to start a reversal on the idea. Is this because it was proven such a colossal failure?

Answer

In Episode IV Obi-Wan claims, "for a thousand generations, the Jedi were the guardians of peace and justice in the galaxy." Taking at this at face value, if their traditions had worked for 20,000 years or more they must have been doing something right. Historical orders of warrior monks, such as the Knights Templar or Buddhist sects in Japan, have been quite successful for centuries.

In Episode I, Yoda believes Anakin is too old to be trained as a Jedi, at the age of nine or so. In Episodes II and III, we see younglings who are no more than four or five. This suggests Jedi begin their training at a very young age indeed, perhaps in infancy. The training (brainwashing?) to reject attachment may be more effective when the recipient starts so young.

That said, in Episodes I-III the Jedi are distinctly arrogant and complacent. In Episode II, Mace Windu asserts it is "not in his character" for Count Dooku to do anything evil, which turns out to be quite wrong. And of course, they fail completely to notice Palpatine's schemes.

As noted in Richard's answer, Obi-Wan seems blind to the possibility of a romantic relationship between Anakin and Padme. At the beginning of Episode III, Anakin is spending the night in Padme's apartment instead of some Jedi barracks, but no one seems to notice. (That, or the Jedi are deliberately turning a blind eye to it, which would be a colossal failure of judgment.) Later, when Anakin is in obvious distress (video in Richard's answer), Yoda fails to show compassion or give any helpful advice, instead just repeating Jedi doctrine at him.

All in all, the Jedi of the prequels are a pathetic lot. Their rules may have served them well in the past, but they are totally inadequate for the case of Anakin. So to answer your question, simply telling Anakin to reject attachment is a terrible idea, but the Jedi don't realise that until it is far too late.

In Episodes IV-VI, I think Obi-Wan and Yoda know this. They are old and haunted by their past failures. For them, Luke is a desperate throw of the dice to try and bring down the Empire.