Because I am an atheist...
I no longer have to struggle to reconcile what I see and experience with what I want to believe is true, or what I'm told I should believe is true. I must evaluate everything on it's own merits, not by comparing it to the requirements of my faith.
I can look at people who are different than me, people who are gay, people who are of different color, people who have different religions, people who have different ideas... and not see them as sinners that need to be saved, but as people who have had different experiences or made different choices. I do not think that making them believe as I do is an act of righteousness, although there are times it may be an act of reason.
I try to squeeze every bit of joy, knowledge, and experience out of this life. I know there is no other, and that some day I will be no more. If I want joy, I need it here. If I want to share my love, I need to do it in this lifetime. If I want to be remembered, if I want to leave a mark, if I want to know that something of me goes on, it must be that I've done something memorable, something that made an impact, something that changes some little piece of the world.
I am responsible for my own actions and my own inaction. Man is not "like an ass, ridden at times by God and at others by Satan" (Martin Luther). Jesus does not take the wheel. Ganesha does not remove the obstacles, and the Goddess isn't going to bless me. There is no divine will guiding my steps, and I can't use it as an excuse for failure, nor can I use it as a reason for success (no, I'm not always right because God is on my side).
I do not wait on the Lord. I know that if I let go of something, there is no God to pick it up. If I chose inaction, I take responsibility for that. And I know if everyone on the highway really let Jesus take the wheel there'd be a heck of a pile up on the interstate.
I don't believe that there's a time limit on human existence on this planet set by God, or that it doesn't matter what we do to this planet because it's not our permanent home. This is all the home we have, and any time limit we have is one we've imposed as a result of our carelessness.
I do not feel entitled to the planet. It was not given to me. This world is my home, and like my home, I don't want to foul it, destroy it, or harm any of the inhabitants. I feel no need for "dominion". I feel a need to be part of it.
I'm not ashamed to be related to a monkey. I'm not ashamed to be a primate, a mammal, an animal. I know that I have kinship with all life on Earth, and that I'm a part of the food web, the circle of life, or whatever you want to call it. I don't think that humans are "superior" in some sort of divine way, nor is our importance any greater (or less) than any other species on the planet, although I recognize that we've developed a greater capacity to impact the health of the planet as a whole than any other species. I'm not sure that says anything about our "superiority", but I'm pretty sure it says a lot about our responsibility.
I'm not dirty, broken, and unworthy. I don't see humanity as a product of original sin, or that we need to overcome some beastial or base nature in order to reach the divine. We aren't born wrong and need to be fixed... not by any spiritual practice, not by the Blood of the Lamb, not by the 8 Fold Path. We are born fully human, fully what we are, and full of potential. We are born ourselves, and remain ourselves... and that's good and right.
Because I'm an atheist, I want to use my limited time in this life in acts of love, learning, and conservation, not in fear of damnation, hatred of the different, and waiting for God(ot).
I'm not worried about going to Hell, even though you may think I may be. You may fret over my salvation. You may worry I'll never reach Nirvana. You may think some day I'll be sorry. I'm sorry you feel that way. The way I see it, some day I'll be dead... And some day so will you, and all that worry will come to naught. And it's OK. I just choose to live my life NOT fretting over whether or not I'm worshipping the right God, praying the right prayers, eating the right foods, celebrating the right days, and so on, when I've no evidence of any of it making any difference. I choose to live out my days in ways that I know matter.