Heck if I know why he's crying. Sure, I'm his mommy and spend practically every waking (and some sleeping) moment with him. If he has a specific cry for hunger or exhaustion or whatever there isn't enough of a difference for me to tell. The only difference I can tell is the "urgent" cry and the "take your time" cry.
So he may be crying because he's tired. Or because he's teething. Or hungry, or wet/dirty (though I check for that first). He may be bored. And it's always possible that he's trying to break my will (and sanity) so that I will carry him through Manhattan.