I agree with the 'positivity is the key to happiness' comment, that's why it doesn't make sense for you to say we can't change things.
Even when you only do something as small as explaining to the woman at table 6 why veal isn't on the menu. Maybe she hadn't known before that.... maybe she won't feel so good bout eating it again.

Do not overlook tiny good actions, thinking they are of no benefit;
even tiny drops of water in the end will fill a huge vessel.

Do not overlook negative actions merely because they are small;
however small a spark may be,
it can burn down a haystack as big as a mountain