Jesus touched on this dynamic of a warning which doesn't seem real until the experience when he told the parable of the Rich Man and Lazarus. Lazarus was a poor beggar who sat at the rich man's gate day after day until they both died. Lazarus ended up in eternity at Abraham's side while the rich man discovered himself in Hell. After unsuccessfully pleading for Abraham to send Lazarus as an errand boy of relief, he pleaded for Lazarus to be sent to his brothers, insisting that they would repent of their evil ways if a dead man was resurrected and sent to them. Jesus' reply through Abraham's voice was, "If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead" (Luke 16:31, NIV). Hell became a motivating factor for the rich man who wanted to warn his brothers, but just as the warning of Hell had not been enough to motivate the rich man, Jesus insisted even a supernatural warning about Hell would not be enough for his brothers. The lesson? Hell should be a motivator for the evangelist, but isn't necessarily a motivator for the lost.
Sometimes we get it backward, trying to warn people about Hell and hoping that they will change their ways, and then dismissing their refusal with a "Well, I tried. If they want to end up in Hell, more power to them." But if we saw an oncoming car about to run over a stranger in the street would we just say, "Oh well, I guess if they want to get run over it's none of my business"? Hell isn't necessarily going to be a factor for the lost. They don't believe. Hell isn't real to them; it's just something someone has told them about. But Hell should be real to us who believe. It should be a motivating factor for us. And it should drive us to find ways to share the gospel with people that amount to more than, "You'll go to Hell if you don't repent." Hell should motivate us to show the love of Christ as fervently and sincerely as we can. Hell should motivate us to tell people about the offer of reconciliation with God and the forgiveness of sins that Jesus' sacrifice made possible for us. Hell should give us an urgency in reaching the lost that we lack far too often. But Hell probably isn't going to motivate unbelievers until they start believing.
Let Hell motivate you. But don't rely on Hell to motivate the people you are reaching. Jesus does a much better job of motivating the lost than Hell does.