I think that just depends on how you define selfless. You'll still get a good feeling from doing something good but that doesn't mean it's not selfless unless you like being super technical. I think those are just our senses and it's how we deal with them that matters. Let's say you have $500 and you know somebody who it could really help but you also know a way you could spend it on yourself. Deciding to give it away instead of spending it on yourself is thinking of others, and your brain just let's you know it was the right choice. It's selfless because in a scenario where you could have picked yourself you instead picked somebody else. It seems weird to me to say because we respond to stimuli that we are wholly selfish beings. Having said that, some people definitely do nice things only for the fact it makes them feel better about themselves. It's just not necessarily the only way to be.