Do not forget to add the line allow-hotplug eth0

/etc/network/interfaces

# Used by ifup(8) and ifdown(8). See the interfaces(5) manpage or
# /usr/share/doc/ifupdown/examples for more information.
 
# We always want the loopback interface.
#
auto lo
iface lo inet loopback
 
#auto eth0
#iface eth0 inet static
#  address
#  netmask
#  gateway
 
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp