If you're a logical human being, your default answer should be no. Any question like that has to begin with the answer no. So, what brings your answer to yes?
If you were asked that same question, said no (like before), but were then provided with proof, it would be okay to say yes. You see, to arrive at the conclusion of yes you need to be able to quanitfy it. Otherwise, you're arriving at the conclusion on pure faith. You wouldn't just say "yes" because someone asked you a question, or told you so, would you?
Now, I want to ask you a different question. Do you believe in God?
As before, the default answer is NO. You need evidence to jump to the conclusion that God does exist. If you don't have any, than you are simply stuck at no. Jumping to yes, without evidence, is fallacious and in this case delusional. Now, don't think that I'm saying that atheism doesn't require evidence - of course it does. What I'm getting at is that the initial claim of "yes" requires immediate evidence, whereas evidence for the "no" comes afterword.
Now, evidence in this case does not necessarily have to be scientific. I think it does, but that is irrelevant. Some people believe that nature itself is evidence for God, and this is an acceptable way for them to justify answering "yes" to that question. Some say the Bible is evidence, and some claim to even have had personal experiences relating to God. Okay, that's fine, your position has been justified with "evidence". Whether or not the evidence is legitimate is a different story.
So, in short, Atheism is the default answer to the question of the existence of God. Not because it's the truth or that it is a better response, but because "no" is the default answer to a debatable question.
Whenever you do a proper debate, the side that is always for something is the one who must provide the evidence (they usually present their case first too). This is called having the "burden of proof", and is very important when approaching philosophical discussions. Just to get my point across, here is a tiny simulation. Group T represents the theists, and group A the atheists.
"Do you believe in God?"
Group T: Yes
Group A: No
Group T, the theists, have the burden of proof so it is up to them to quantify their jump to the "yes" conclusion. Group A then makes statements in rebuttal, offers evidence, and the cycle continues.
The ultimate point that I am getting at is that I am an Atheist because there was never any evidence to move me to become a theist. This is why I see theism as a delusion, because the only evidence comes from things that are not quantifiable and cannot be tested. On the other hand, we know many of the natural processes that make the world work without invoking a god, so why do it in the first place?