1. You don't move from your TV spot...
2. You become emotionally invested and connected to all the characters i.e. their pain is your pain.
3. You try to spread the word to everyone about how good it is so you have someone to talk to about it.
4. Cliffhangers kill you inside.
5. When the series ends you go into a mild...well, big depression and then proceed to rewatch it again and again.