The link between gratitude and self-control
Gratitude encourages people to repay debts.The more gratitude people feel toward those who have helped them, the more diligently they will work to return the favour.
When you want to repay someone you have to forgo your own immediate needs in service of someone else. This action boils down to self-control.
For instance, when you are grateful that a friend helped you to move to a new apartment, you are more likely to return the favor, even if you have to forgo something you looked forward to.