Jesus would always seem harsh against the ones that did not want the things of God. The religious rulers took the biggest hit on this. They would show outward holiness by keeping the law, while on the inside of them there was nothing that even desired the things of God.
Jesus would also seem harsh against Israel at times. He would call them an adulterous generation. Adulterous, because they had the Messiah right in front of them, but they did not accept him. They continually sought after other gods, ignoring the one true God who came in the flesh.
The unbelieving crowd was another group that Jesus seemed harsh against. The disciples fell into this group quite a bit of the time. The disciples were with Jesus 24/7, but they would constantly forget what he could do or has done. There were many miracles done just so the disciple could see first hand that Jesus was who he said he was. The feeding of the five thousand was one of these events. Each disciple got a basket of breadcrumbs to carry afterwards for a reminder.
Some might think that it is harsh to mention that there is an actual hell. Hell is a real place. This place is an eternal separation from God. It is a terrible place, because the one that goes there will be in their physical body that can feel pain and worse than that, they will remember the opportunities that they had to be saved from it. No one spoke more about hell than Jesus did. He would mention how there will be weeping and gnashing of teeth in darkness. He spoke the most about it, but also he did the most to keep people from going there, by dying on the cross and being resurrected from the grave. He beat sin and death and now provides eternal life to those who believe. Because of what Jesus has done on the cross, eternal spiritual separation from God (i.e. Hell) can now be avoided.