Suits of Armour
Not all armour is visible. The strongest armour is the one we use to protect our hearts.
Jack's armour is an act, a grin and a devil may care attitude. If he pretends it doesn't matter long enough maybe one day it really won't.
The Doctor's is a manic energy, a restlessness and constant chatter. If he keeps running, if he surrounds himself with noise and activity then maybe he won't hear the silence in his head.
Ianto's is his calmness, his steady methodical nature. If he can destroy the outside traces of fear and grief and pain then maybe one day he can destroy the feelings themselves too.
Donna's is her abrasive nature, her mouth and sarcasm. If she can act confident enough maybe she can hide the self doubt gnawing away at her, telling her she will never deserve to be loved just for who she is.
The longer you wear your armour the harder it is for you to take it off, to let anyone inside. But when you find the right person they can find the chinks, strip it from you piece by piece. Until they find the real you. Only then can they heal or break your heart. The problem is you won't know which it will be until you let them in.