Well-intentioned people sometimes attempt to resolve the marital problems of their friends.
Conclusion (Thus)
Even well-intentioned attempts to resolve the marital problems of friends are usually unjustified.
Evidence
Well-intentioned attempts are usually ineffectual, thereby fostering resentment among all parties.
Evaluate
The judging term we're trying to prove in the Conclusion about well-intentioned attempts is "unjustified", which can be expressed in synonymous language like "bad / should not do / not right".
The ammunition we have from the Evidence about well-intentioned attempts is that they're "usually ineffectual and thereby foster resentment among all involved.
Goal
So, if we're anticipating the typical "IF premise, THEN conclusion" linking answer that's so common on Principle questions, we'd want something like
ineffectual → unjustified
or
fosters resentment among all parties → unjustified
The answer should basically attach something we were told is true of "well-intentioned attempts to resolve others' marital problems" to language that sounds like "unjustified".