Faith Well Grounded

Why is it so easy to fall into the "us against them" mentality? I see this a lot in Christianity-- especially apologetics. We see those who oppose the gospel as infidels, enemies to God, those who we must refute! Why is it so against our human nature to see them as those most in need of God? Why is it that so many refuse to show God's love to those who most need to see it?

Rom 5:8 but God shows his love for us in that while we were still sinners, Christ died for us.