Are you looking for moral arguments, or expecting moral arguments? Because there's a difference between being a good person and doing good acts, and lots of non-good people do good acts all the time for lots of reasons, morals be damned!
They like to look like a good person (tons of social benefits), they like to FEEL like a good person (ego benefits), they use the good to offset the bad and can use it to get favors/repayment from other people (monetary or utility benefits), they do good deeds to help others that are in their family or friends circles (companionship benefits). There are literally tons of purely logical reasons to do good things, ESPECIALLY if they're at no cost to you.
Lots of people automatically assume extreme selfishness is the smartest choice, though.
(ETA: That's why so many sociopaths are smiling, charming people - if you really didn't care about people, you'd use them, and getting them to trust and like you goes a long way. Much, much further than hating them and thinking they're all scum, which only hurts you by alienating you.)
(ETA2: I am not a sociopath! I have just studied them)
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They like to look like a good person (tons of social benefits), they like to FEEL like a good person (ego benefits), they use the good to offset the bad and can use it to get favors/repayment from other people (monetary or utility benefits), they do good deeds to help others that are in their family or friends circles (companionship benefits). There are literally tons of purely logical reasons to do good things, ESPECIALLY if they're at no cost to you.
Lots of people automatically assume extreme selfishness is the smartest choice, though.
(ETA: That's why so many sociopaths are smiling, charming people - if you really didn't care about people, you'd use them, and getting them to trust and like you goes a long way. Much, much further than hating them and thinking they're all scum, which only hurts you by alienating you.)
(ETA2: I am not a sociopath! I have just studied them)