“Think of the worst you have ever felt in your whole life—like if you got a bad flu or your dog died or you just got cut from a team you really wanted to be on—and imagine that happens when you take nine steps to the bathroom instead of ten.” 

Daniel is describing the pain he feels living with OCD anxiety. This has two aspects. First, it is extremely intense and unbearable. Secondly, it is ostensibly irrational, based upon doing things a certain “wrong” number of times.

“It was like she was the only person who had ever actually seen me.” 

Daniel is referring to Sara, after she speaks to him for the first time. He feels an immediate connection with her, which is seemingly based on the semiconscious sense that she knows something about what makes him different.

“He had turned it on, and he had done something terrible.” 

In the opening of Daniel’s story, fictional Daniel flicked a switch on a computer in the attic and wiped out the human race. This is a metaphor for Daniel’s OCD anxiety, and the fear that terrible things will happen if he doesn’t “fix” certain things.