One of my two favorite books is East of Eden by John Steinbeck. (The other is The Brothers Karamazov by Fyodor Dostoevsky...ironic maybe that both books are about brothers? weird.)
It's a book about free will, about choices...about how what you choose is who you become, or defines who you are. One of my favorite passages in East of Eden (actually in any book, ever) describes the differing translations of a single Bible passage, and the implications of what the different translations of that passage mean. The word around which the translations vary is "timshel," which literally means "thou mayest."
This is the core of what it means to be human, the central guiding factor of anything we do. "Thou mayest." You choose what you do. You choose who you are. Each decision that we face, we have the opportunity to choose our course of action.
There are implications of your choices, for yourself and for others, but ultimately you need to decide who you are deciding for. Is what is good for you and bad for others a good choice or a bad choice? Is what's good for others but hurts you the right decision? Who defines what is a good or bad choice? Who decides whose happiness is more important? Who decides whose pain is more valid? Who decides who is right or wrong?
The truth is, we all do. We, each one of us, decides every day. Luckily, most decisions aren't that complicated...most decisions don't have serious implications for our or other peoples' happiness or wellbeing. But some do. Do you stay at work late to work towards a promotion to make more money to support your family, or do you leave early to spend more time with your kids? Do you end a relationship that isn't working and deal with the consequences, or do you fight to make it work even if it never turns out to be something that makes you truly happy? Do you forgive a violation of your trust, or are some trespasses unforgivable? Do you keep the $20 you find on the street, or do you turn it in? Do you make decisions for the sake of your kids, or do you make decisions that set an example for your children? If you do things for yourself, are you selfish? If you do things for other people, at the expense of doing things for yourself, is that better or worse? Do you judge and treat others harshly for decisions they make that are different than your own, or do you accept our differences as we're all trying to figure this out, day by day?
You have a choice. You decide who you are. It's empowering. It's terrifying. It exposes weakness. It shows true character. It provides opportunity for redemption. It is the core of humanity.