When you install Wireshark on Ubuntu Linux you need to be root to be able to capture traffic. The standard user doesn't have enough privileges to do this.
A workaround for this is to add the user to a wireshark group and give the group special permissions. Afterwards, you're able to cpature traffic in Ubuntu with Wireshark, without needing root access.
The complete list of commands:
sudo groupadd wireshark
sudo usermod -a -G wireshark <YOUR-USER-NAME>
sudo chgrp wireshark /usr/bin/dumpcap
sudo chmod 750 /usr/bin/dumpcap
sudo setcap cap_net_raw,cap_net_admin=eip /usr/bin/dumpcap
sudo getcap /usr/bin/dumpcap
Just reboot, or log out and back in, and you're finished.