Why does Eli dress like a girl?

[sauvin]

I'd like to think that Eli didn't pick Oskar only because he was 'An incontinent liar and thief who keeps a scrapbook of murders and harbors fantasies of bloody revenge,' although that was certainly the ice breaker.
I think it was because his own dark life made it easy for him to embrace who Eli really was (or wasn't) and damn the rest of us.

There's a very old saying, forgot in which language: "tell me who you walk with and I'll tell you who you are". I suppose an English language way of saying this would be that "water seeks its own level".

Who chose whom first? I seem to recall from the novel that Oskar's first reaction was curiosity piqued by an unexpected moment of what he would have taken as intimacy, something he may have never experienced before with someone outside his family. Eli's reaction to that moment was a startled confusion; she probably had experienced this kind of intimacy (i.e. absent adult predation) at least once in her long past, but apparently the memory took a while to bubble up to the fore.

The movie swerved here a bit, I think, because that startled confusion would have added some dimension to their nascent "against the odds" relationship that's missing when she brusquely states "Just so you know, I can't be your friend. That's just the way it is." The novel underscored her Otherness brilliantly here.

Using the word "choice" here is a bit loose. The only children her own age she might have met at any other time or place, if they were "normal", would have eventually been repulsed on moral or "ick factor" grounds once having discerned her condition, and most not-so-normal kids would would have been "off" enough to make growing a genuine relationship problematic at best. It would have done no good to point out to Oskar that he probably would have eventually met somebody he could spend some real time with; at twelve years old, a year or six into the future is every bit as lost on the horizon as a century or six.

They chose each other because the one resonated with the other, they chose each other because not doing so meant living with nothing, and they chose each other because the one seemed to be the best humanity could offer the other.