I'm an atheist because....
1) The traditional notion of god isn't coherent
In order to even consider the possibility that a god exists, we first need a coherent concept of god. The traditional notion of god in classical theism is that of a timeless, changeless, immaterial mind, who also must be infinitely good, infinitely wise, and can do anything logically possible. There are some variations on this concept, but almost all traditional or classical theistic gods have these basic characteristics. The problem is that a timeless, changeless being by definition cannot do anything; it's necessarily causally impotent and nonfunctional. Change requires time, and time requires change. This is logically certain. And to create something, one must do something. Doing requires a change, regardless of whether that change is mental or physical. A being that cannot do anything cannot be omnipotent. As a result, the traditional notion of god is self contradictory. The theist's only resort here is special pleading. That's why I like to get all theists to agree beforehand that god is not beyond logic. That is, god cannot do the logically impossible or be the logically impossible. Once a theist agrees with this, they've cut themselves off from special pleading as an option. Some theists think god is atemporal before creating the universe, and temporal after creating the universe. But it isn't logically possible to exist timelessly and then suddenly jolt yourself into time out of your own will, because your will was timeless and frozen. It couldn't change into the state to want to change.
Given the necessary rules of logic the traditional attributes of god are incoherent:
P1. It is logically impossible to do something without doing something.
P2. It is logically impossible to do something without change (even if everything is immaterial).
P3. It is logically impossible for change to exist without time.
C. As such, a timeless, changeless being cannot do anything.
The failure of theists to come up with a coherent description of god is enough by itself to warrant atheism, but there's many more reasons to think no gods exist.