Your acts of kindness and mercy is not a favor to G_d that would merit a reward or blessing but should be treated as payment for your past transgressions and as a form of gratitude to the past blessings He has bestowed upon you.

It is your gratitude through your acts of kindness that will open the windows of heaven for your blessings.

Let your motives of kindness be moved by your gratitude because,

'In the eyes of the sovereign Judge the merit of our actions depends on the motives which prompted them.'

(Pope St. Gregory the Great)

G_d does not need favors from you. It is actually the opposite, you need favors from Him.

Every blessings you receive today is not yet paid. Every kindness you do today is your payment for His past mercies.

In that sense, you could not do enough to repay G_d of His kindness yet in His majesty and love He renews your blessings everyday.