When people hurt, they long to feel understood. Even when they bring problems on themselves, they seek out sympathy...or at least acknowledgment and validation.
Certainly, there comes a time for compassion to lead to confrontation. We mustn't coddle sin, lest we lead the person further into the sin of self-pity. At the same time, Scripture commands that Christians deal with another person's physical or moral weaknesses by demonstrating attitudes of gentleness and forbearance.
Brothers, if anyone is caught in any transgression, you who are spiritual should restore him in a spirit of gentleness. Keep watch on yourself, lest you too be tempted. 2 Bear one another's burdens, and so fulfill the law of Christ. 3 For if anyone thinks he is something, when he is nothing, he deceives himself. ~~Galatians 6:1-3 (ESV)
If I'm honest, I'll admit that I'm too selfish to listen to someone else's chronic venting. I'd much prefer to shift the conversation to my supposed expertise on the matter, so the person can gratefully thank me for solving their problem with my abundance of wisdom. That way, I get to bask in the notion of my spiritual superiority, reminding myself (and perhaps others) of my important role of dispensing God's wisdom. All the while, I grieve the Holy Spirit with my insensitivity to broken, hurting people.