To be kind of douchey interjecting amateur psychologist -- emotion can also be influenced by your brain just plain fucking up and throwing a gear. Sometimes the brain has no reasons to do what it does -- it's just programmed to, whether or not that programming is advantageous to you, or even if it's responding to a specific stimulus.
Um, I hate to be the Christian to come and rain on your parade... But, this is obviously a topic that religion is going to come into play quite a bit.
Here's, basically, what my belief system says about this topic;
Humans have a sin nature. The only way that they could do an absolutely unselfish act was if they were forced to.
Jesus, however, was a human and was perfect. He completed his life without any sins. So, he did not commit a single selfish act. Selfish, here, meaning "vain and self-centered." This would make it a sin, so obviously, if you believe as me, Jesus fulfilled unselfish acts several times.
Now, here comes the exact wording... "A human can".... The Bible says that there will not be another Messiah, and since humans have a sin nature, I don't believe they can. I believe that they could... Er, one could.