Why are barre chords so hard to play?
Playing a full barre chord involves using the index finger across the entire set of strings. What makes this difficult is the tension of the strings and the dexterity needed to hold them all down at once to produce a chord that sounds clean and clear.
Will barre chords always hurt?
They really shouldn't have to hurt that much. Maybe your technique is off or your guitar is set up badly (the higher the action, the harder bar chords become). It definitely does, I've been playing for about 10 months and my hand only starts getting tired after playing my acoustic for a while (high action).