A group becomes more effective when disagreement can improve an idea without humiliating its author.

Ideas stop arriving when proposing one is risky. If disagreement in a group routinely comes with a small public cost to whoever offered the thing, people learn to bring only safe suggestions, and the group loses the half-formed ones that would have been most useful. Separating an idea from its author is not politeness for its own sake. It is what keeps the supply coming.

In the next discussion you join, argue with the proposal and speak well of the person who made it. Say what the idea gets right before you say what it misses, then invite them to fix it themselves.