Why Would Users Hack Into a Network Without Authorization?

In my school there is a Firewall installed between its data and unauthorized users, it protects its network by preventing possible viruses, spyware, and users who just might want to make a mess of things.
I think a user might want to hack his/her way into our network:
-to access another persons (teacher,student) files and make possible changes
-to gain free internet access
-to gain access to the network administrator
-to install a virus or type of spyware software into our network
-to download software from our network
-to gain access to other workstations that may have access to banking accounts
-just to screw around