Well, first of all, the rule IS saying you are obligated to 'do unto others as they would be done unto.' That's.. the whole rule. You can quibble over whether 'obligated' and 'should do' mean different things, but that's semantics. If you 'should' do x you are morally obligated to do x, that is the meaning of 'should.'
Secondly, just because someone is sick and wants something that is bad for them doesn't mean you ought to give it to them! Whether that is a beating, or meth, or a bullet to their head... or a loan that will ruin them... or to buy all their possessions at a pittance for them in their time of desperation... or to lead them somewhere they want to go even if you know it will harm them... that doesn't mean it's right. it doesn't matter if the desire is genuine or not genuine, it doesn't mean you ought to do it, as in this rule.
The rule loses all value because there is simply no reason why anyone would go around doing unto others whatever the other happens to want. Few people manage to follow even the golden rule, of treating others as one would want to be treated oneself. But at least that rule has some basis in a self referential standard, something we can actually know and apply. Cases of masochists and other mentally aberrant individuals don't really figure into the equation, and apply with equal confusion in either rule.
Finally, the rule is a rule for how to treat people you don't really know. We already have ample motivation to treat family and friends well out of love. The golden rule is a rule for people we don't know. We could hardly do unto strangers and people out on the street as they wish to be done unto, since we don't know what they would want from us, empathy or no empathy. We have to go with something we know, how we'd wish to be treated. Or some other personal standard of right that we hold to.
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