Gratitude! No, he definitely couldn't let Lisbon know how grateful he was to her.
Not just for the big things – springing him from jail by using her influence over Bosco, punching out Donny Culpepper and enduring another blot on her record and those wonderful anger management classes – but for everything.
For caring. For being his friend and confidant. For taking on a wreck of a man and keeping him afloat in his despair.
He had a very good idea of just how much he owed her.
He hid it behind teasing and banter. Loyal to her till death.