Opposite of disappointed

The opposite of like is dislike. The opposite of assemble is disassemble. So here, “dis” is a prefix that acts as a negation.

Why is the opposite of “I’m disappointed in you” not “I’m appointed in you”? Is the “dis” actually a prefix or not?

According to Oxford dictionaries, the derivation of disappoint is -

late Middle English (in the sense ‘deprive of a position’).

So I suppose the meaning of disappointment has extended over the centuries from the feelings of someone who didn't get a job they had wanted, to cover all kinds of disappointment. People sometimes amuse themselves by thinking of all the English words that exist in the negative form but no longer in the positive, like unkempt, uncouth and disgruntled.

Oh, and disgust comes from the French dégôut (distaste).