It is like in the ancient war, the chosen ones are not to lust over their spoils in their victory. Their only desire is to fulfil God's command, not to stand over the bodies of their enemy and boast their strength.
Gratitude should be next to mortification of sin. When one prays for deliverance from sin, how is it right to be proud or seek after such attitude when the sin is 'overcome'? Does such not prove that the sin is not conquered by mutated into another form, a blind substitution?
Thus, when an ex-gangster, porn-addict, depressed, etc. shows no sign of gratitude when praising God for their former faults, but as it those sins were somewhat a necessary part of their spiritual growth (oh what blasphemy!), they have merely turned into different sinners and more stubborn.
A repentant soul would be as thankful to God as one being delivered from a great enemy and he would have a regret that made him wish he hadn't done it, not out of pity on something/someone nor so that he could boast, but out of the thought 'how could he have been so stupid, so lazy, so selfish and so ungodly'. And then be peaceful in his new life in Christ without feeling ignorant or jealousy on those who had done better, but admiring them, praising God for them.