I think your argument is strange. Disappointment may inspire change, but the want for change comes from the inherently negative nature of the disappointment. Saying that disappointment is not so bad because it makes people want to change something is like saying the murder of Emmett Till wasn't so bad because it inspired such great civil rights improvements. Positive change is good, but I don't agree that it necessarily reflects upon its inspiration. Disappointment can have good effects or bad effects or no effects at all, but whatever it does or does not inspire does not change the nature of disappointment itself as an inherently negative feeling.

As for whether looking positively on negative things is a positive or negative outlook - that's just word bending. It is what it is.