Reply with the end of the sentence, and when everyone has replied at least once, then start with a new one.
Here's the first one (and I'll make my answer too, btw):
"You know you love Lemmings too much, when ______________."
My answer: "you wake up in the morning with one of the bgm tunes stuck in your head." (I did that this morning, actually.)
...you get run over by a bus because there was no blocker stopping you from crossing.
You start to notice that everything on earth must be one of nine main themes. (i'e) marble, fire, sand,cave,earth,bubble,rock,snow,brick
...listening to
certain pieces of classical music makes you think of exploding green-haired rodents.
You adopt a real lemming as a pet... and dye his fur green