Are you seriously going to pretend that you aren't selfishly motivated? That you've somehow managed to remove yourself from hundreds of thousands of years of the evolution of moral reasoning and social behaviours, and now operate in a clean, mathematical moral vacuum? Even the way you've spun this strawman is selfish toward you. Have you really exhausted all the other possible explanations for your moral motivations? Have you considered the possibility that the reason you do good things, isn't 'for the mere act of doing them', but that because you are too cowardly to do bad things (ie. that you fear punishment from others), or that you do good things to be rewarded in some sense (ie. being left alone, gaining social approval and/or upvotes). If not, you might want to reexamine your reasoning - and less in your own favour.

Sorry to blast up about this - I'm an atheist too. But I hate seeing a pot call the kettle black, or however that expression goes. We're all damn humans. Let's be more honest.