I think that's an artifact of Christianity. The 8th commandment explanation in the Lutheran (and probably Catholic) catechism seems to imply that one should not only avoid lying about other people, but "go the extra mile" to say only good (or at least constructive) things:
The Eighth Commandment
Thou shalt not bear false witness against thy neighbor.
What does this mean? We should fear and love God that we may not deceitfully belie, betray, slander, nor defame our neighbor, but defend him, speak well of him, and put the best construction on everything.
The italics at the end are mine.
Understanding Catholic--and Lutheran--doctrine, the emphasis is on doing good works to counteract all the bad works in the world. Think of it, outside of Christianity, as raising the world's karma level (and yours, in specific.)
If you look at it generically like that and in a practical sense, it might be a good idea. I don't try very hard
good at it. I think people who behave that way gain more respect though. For example, a salesman who talks down the brands he doesn't sell probably doesn't get as much respect from you as one who recognizes the strong points of the brands he doesn't sell, but emphasizes superior characteristics of the brands he does sell.
added strike-through