Is it incumbent upon a person to be keen on returning favors and kindness whenever possible, or how can one reward the doer of kindness?

The Prophet, may Allah's prayers and peace be upon him, encouraged rewarding those who do good deeds. If a person cannot find anything to reward them with, they should supplicate for them, such as by saying: "May Allah reward you with goodness" (Jazakallahu Khairan). Whoever says this has indeed expressed the utmost praise. As narrated by Al-Tirmidhi, the Prophet, may Allah's prayers and peace be upon him, said: "No one has a favor over us but that we have recompensed him, except for Abu Bakr, for he has a favor over us for which Allah will recompense him on the Day of Resurrection. And no wealth has ever benefited me as much as the wealth of Abu Bakr."