One of the starting points in understanding the need for the Gospel is in understanding the reality of what happens if someone rejects the Gospel, because if we don't know where we're going, then we have no reason to change our course. And it's an unfortunate doctrine we have to discuss, because it's unpleasant in every possible way.
The reality of Hell is one of the things Christ repeatedly spoke about. Many of His parables tell us about people being cast into "outer darkness" and "weeping and gnashing of teeth". Perhaps the strongest words on the subject come from the parable of the sheep and the goats, with the goats going into eternal punishment and the sheep going into eternal life.
At this point is where we get introduced to universalists.
Let me say this clearly: universalism is not Christianity. Anyone claiming to be a Christian universalist is deceiving themselves as much as a person who claims to be both a poached egg and a human being. These simply are not compatible belief systems. Universalism ultimately teaches that Christ's sacrifice was as unnecessary as your need to accept the Gospel because you're going to Heaven, no matter what you believe.
What a hopeful belief system! If only it were true.
Seriously. I wish it were. I wish that scripture taught that everyone would end up in Heaven in the end. The thought of someone spending eternity seperated from God is a sickening thought to someone who understands the concept of Hell. No one would ever celebrate someone ending up in a place describe by scripture as a burning, stinking garbage heap that never extinguishes, where the smoke of their torment ascends forever and ever. What a horrible concept Hell is.
Unfortunately, it's also a very real, very true concept - and it's that concept that should drive us to preach the Gospel. Sure, we can talk about Jesus loving us, but let's be honest: we can get love elsewhere. We can get our needs met elsewhere. We can get everything we could possibly need elsewhere, whether or not we're Christian, Buddhist, Islamic, or what have you.
But we can't get salvation. We can't get Grace. We can't get the Gospel. These are the things that we so desperately need, and these are the things that we must be driven to. This is why Hell is so important to the Christian - not so that we can condemn people we don't like there, but so that the reality of it will drive us to understand the consequences of someone's decision to reject Christ, so that we will be emboldened to evangelize, preach, and proclaim Jesus Christ crucified and risen again.