One good turn deserves another
Definition 1 of 1
Pro Tip 1/3Reciprocity
This means you should return a favor. It's about mutual help, not just feeling thankful. 
He lent me his notes whee I was sick. Now he needs help moving - one good turn deserves another.
Pro Tip 2/3Expectation
Using this phrase can imply an expectation of help in return, not just a desire. 
You took my shift last week, so I’ll take yours today - one good turn deserves another.
Pro Tip 3/3Emotional Tone
The phrase often carries a positive tone suggesting kindness and friendship. 
She always supported me, so I’ll sponsor her event - one good turn deserves another.