Currently nethogs defaults to eth0. This is OK but it would be better if it defaulted to the default interface in the routing table. It's a bit tedious on laptops, for instance, where the wireless interface is often eth1 or wlan0 and you have to check which it is.
route | grep "^default"
gives you the default interface, though there is probably a better way of picking it up from somewhere in /proc. In fact, it looks as though it's in /proc/net/route, and you'd want the line where Mask=0.
Failing that, using the first interface that's up would be a better choice than always plumping for eth0.