In Fight Club, when Ed Norton first confronts Marla, why doesn't he tell her about his insomnia?

The beginning of Fight Club is all about how Edward Norton can't sleep, and his doctor refuses him sleep medication and instead refers him to go to the support groups at the local Church instead. So he pretends to be a part of all of these groups, and he sees true despair and can sleep again. When he finds Marla, another faker, it ruins it for him and he can't sleep again. When he finally confronts her, they have this conversation:

Norton: Now look. This is important, OK? These are my groups I've been coming here for over a year.

Marla: Why do you do it?

Norton: I don't know. When people think your dying they really, really listen to you instead of just--

Marla: --instead of just waiting for their turn to speak?

Norton: Yeah...yeah.

It is a nice conversation with the interesting piece of profundity about waiting for your turn to speak, but why doesn't he just admit off the bat it helps him sleep? Is he embarrassed? Is it no longer just about that for him? What am I missing?

Edit: It wouldn't be embarrassment since he presumably spent the last year baring his soul to strangers.