If morality is only mental dispositions, then if I value something, it is necessarily morally valuable to me.
Morality is only mental dispositions.
Therefore, if I value something, it is necessarily morally valuable to me. — Terrapin Station
Well that's a weak argument and its first premise is garbled.
Note, you need to say that if morality is YOUR mental dispositions, otherwise the second bit simply isn't true and the premise is false. Furthermore, you need to identify moral values with your valuings, or again the second bit won't be true and the premise is false.
So, basically, for the first premise to be true, it needs to be the same as my first premise. YOu know, the one you kept telling me I hadn't written properly!
That'll make premise 1 undeniably true.
The problem, however, is that your argument has a conclusion that conflicts with the self-evident rational representations of most of us.
Now that doesn't mean it is false, but it is prima facie evidence that it is and so your premises need to be powerfully self-evident to justify drawing it.
Your first premise is true (well, it is if you adjust it so that it is). But your first premise is the same as mine.
Your second premise, however, is demonstrably not true.
Why? Well, first it has no support from reason. That is, it is not a self-evident truth of reason that moral values are your mental states. I mean, how many reflective people get the impression that moral values are 'your' mental states? Er, none at all - not even you, I'd wager.
So, it has no evidential support whatsoever. And in combination with a true premise - premise 1 - it leads to a self-evidently false conclusion.
That means all you've done is demonstrate the falsity of your second premise. Good one!!
And yes, you can construct these arguments for anything, but when you do so it becomes apparent what you need to assume to get to your desired conclusion - and if you find that what you need to assume is self-evidently false, or has no support from reason and entails something that is self-evidently false, then you've discovered that your assumption is false and that you need to revise your conclusion.
It's called 'reasoning' and most people find it incredibly irksome and so they don't bother doing it and when another does it and proves them wrong about things they care about they execute the one who did it.