Myth:
Atheists are just rebelling against their families and acting out to get attention.
Response:
Most people acquire their religion through the influence of their families — whatever religion one's parents and siblings belong to, that is the religion a person learns about from their earliest years and to which a person professes adherence through most, if not all, of their life. It may thus seem reasonable to conclude that anyone who deliberately abandons the religion of their parents must be "acting out" and rebelling against their family.
Just because this initially seems reasonable does not mean that it actually is, though. It is certainly true that when a person rebels against their family, there is a common tendency for them to reject some of the things which are seen as constituting the family's identity. That is, after all, one way to clear the table and seek out new means for constructing one's own personal identity.
Among the things which a person might abandon in such an effort is obviously their family's religion. There is simply no question but that a person's religion often constitutes an important aspect of their personal identity — not just in direct ways, but also in the many indirect ways that religious beliefs and values influence how we approach new situations, how we treat others, and how we make moral decisions.
Thus, the existence of people who abandon their parents' religion as part of the process of rebelling against their family is unremarkable. Those people are not all atheists, however. Many adopt new religions — sometimes closely related faiths, but sometimes also very different faiths from unrelated religious traditions. Just because a person rebels doesn't mean that they become an atheist and abandon theism or religion entirely.
In addition, it's far more common for atheists to report that their road to atheism was hard precisely because it pulled them away from their original identities and the faith of their families, not that they used atheism as a means to accomplish this. Instead of trying to rebel against their family's religion, they instead feel almost pulled against their will to question, criticize, an then finally reject that religion because of the weight of evidence. It's not uncommon for atheists to wish that they could still believe because they miss some of what they have lost, but belief has become impossible because they recognize just how unreasonable and unjustified their earlier religious and theistic beliefs were.
To be fair, there are some who, in the process of rebelling against their families, also abandon religion and theism. Some of them only claim to be atheists in order to shock others — at most, they might have been angry or apathetic theists. If they continue to actually believe, they are not atheists. Others, however, are genuinely atheists and their initial reason for giving up belief in God and the religion of their families was rebellion. Perhaps that isn't the best reason for such a move, but this doesn't matter a great deal now. It might have been reasonable to criticize that move back when it was made, but unless the person is still just rebelling against their family, it really doesn't matter that much.
Depending upon how much time has passed, the chances are against this still being the case — it's much more likely that after becoming an atheist, they have given a lot more thought to religion and theism and is now an atheist for much better reasons. Even if it were legitimate to critique a person's atheism on the basis of their motivations, you wouldn't be able to do this when those motivations that haven't existed for many years.
The truth, though, is that a person's motivations are irrelevant. When a theist relies upon this as a means for criticizing atheism, they are committing the Genetic Fallacy — a form of ad hominem logical fallacy. The origins of a belief or idea have no bearing on the truth, reasonableness, or justification of that belief or idea now. Bad ideas can have good and reasonable origins while good ideas can have bad an unreasonable origins.
If atheism is wrong, it's not because of anyone becoming an atheist out of rebellion against their families; if atheism is true, that's not negatively affected by anyone becoming an atheist out of rebellion against their families. Thus this question is ultimately irrelevant to the most important issues and is only brought up as a means of distracting our attention from those issues — and perhaps because of a person's inability to address them in a more substantive manner.
It should also be noted that the idea that an atheist is "just rebelling" is actually rather disrespectful — it assumes that the person doesn't have rational, sound reasons for disbelieving in gods. Indeed, it is often used as an ad hominem argument against atheism, assuming that by imprecating the character of the person, then atheism itself will be undermined. The motivations and circumstances of any particular person neither validate nor undermine the things that they believe: ideas must stand or fall on their own because of the arguments which can be used in support of or against them.
