Or add the below line above the iface line for eth0 in /etc/network/interfaces:
allow-hotplug eth0
This is how my /etc/network/interfaces file looks like:
# Wired adapter #1
allow-hotplug eth0
iface eth0 inet dhcp
# hwaddress ether # if you want to set MAC manually
# pre-up /sbin/ifconfig eth0 mtu 3838 # setting MTU for DHCP, static just: mtu 3838
# Wired adapter #2
#allow-hotplug eth1
#iface eth1 inet dhcp
# hwaddress ether # if you want to set MAC manually
# pre-up /sbin/ifconfig eth0 mtu 3838 # setting MTU for DHCP, static just: mtu 3838
# Wireless adapter #1
#allow-hotplug wlan0
#iface wlan0 inet dhcp
# wpa-ssid SSID
# wpa-psk xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
# to generate proper encrypted key: wpa_passphrase yourSSID yourpassword
auto wlan0
iface wlan0 inet dhcp
wpa-ssid sharksInThePool
wpa-psk xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
# Local loopback
auto lo
iface lo inet loopback