The “thing” about “fall[ing] half in love” is that “you never know where you are” and it feels as if it is an “atomic bomb” in which it explodes.
You are either in love or you are not in love, there is no half way there and I think that Holden is trying to push himself to falling in love so that he can feel something again.
Holden feels as if falling in love will make him feel something and will replace that void Allie has left, but in reality the idea of falling in love with someone you do not really care about will hurt you more.
Holden is “sort of glad” he is “girls” “drive [him] crazy”