Dictionaries say merely means only, but doesn't merely have a sense of emphasizing the smallness of what the words after it says more than only?
Kindness is not merely for the sake of others.
Kindness is not only for the sake of others.
The above sentences are ambiguous but they continue and actually mean that good deeds without expecting anything in return is the way we should take. If that's the case, then "only" fits it better, doesn't it?