It has been suggested that some atheists are only about making others feel insecure and unhappy. And while it is impossible to speak for all atheists (because we are an extremely individualized and varied group, beyond lacking a belief in a god) I will explain my views.
For one, I really don't believe that this is the primary motivation of the modern secular movement at all, even if there are a few mean spirited people out there. There are plentiful examples of mean-spirited believers. Just go look at a list of Hate Groups (e.g., Southern Poverty Law Center: Ideology) and you will find plenty of hate groups based on religious tenants. I don't see a single group on that list that claims to or even appears to hate others simply out of their lack of belief in a god.
So, while some atheists may be openly hostile towards some religious beliefs, they do so on an individual basis -- whereas, there are ample sources of hate speech in the religious texts of every major religion. Atheism does not command, on pain of eternal torment for disobeying god, that someone sacrifice their own child (as god did with Abraham), that parents should stone their children to death, that because god has given us a parcel of land we must commit genocide and infanticide against every resident of that land that stands in our way, or that we should stone our wives to death if we find they are not a virgin. But the bible contains all of these things.
And while I'm glad that not every member of these groups do these disgusting things (any more; if the bible is accurate people USED to do them), the fact is that they are still used by others to fuel hatred. The fact that many religious adherents ignore or make excuses for these things doesn't make the resulting problems go away.
Can you imagine if atheists had a book that said to go out and murder people like the bible does? The hypocrisy and double-standards are just at ridiculous levels.
Most outspoken atheists are simply about protecting our rights and, in some cases, simply doing what IS right. Atheists generally take issue, not with someones private beliefs (again, I'm sure there would be an occasional exception), but primarily with their actions.
Far too many 'believers' want to use their beliefs as an excuse to deny rights to people they simply don't like or don't respect (most often lately, women, children, and gay people). They have spent billions of dollars fighting against scientific education in the schools (because they know what happens when children are well educated - they know what it means that 93% of the National Academy of Science members do not believe in a god). They have spent billions fighting against gay rights. They have spent billions brainwashing people to fear condoms (not just teaching the 'virtues' of abstinence but actually telling people that "Condoms CAUSE Aids") which has resulted in millions of people suffering and dying, primarily in parts of Africa (which is compounded there by other, unfounded, magical beliefs). They don't just try to help the poor -- they preach that being poor is a virtue, and they trade their help for allegiance to a flawed and evil doctrine. This is called exploitation.
Just compare the typical Atheistic billboards "You know it's a myth", "Millions are good without god", etc with these:
One claims that non-belief is Treason! They are suggesting we should be put to death because we find the idea of their child-murdering, slavery-endorsing, misogynistic god offensive to our sensibilities of decent behavior.
To me, there really is no comparison between those who are merely being outspoken against religion and the religious treatment of others (just look at the many wars fought between different 'Christian' factions - much less what they have done through history to non-believers and those of other faiths). I don't mean to only pick on Christians (the other major religions are absolutely no better), but they are the sect I know best because I live in the south and have to deal with their hatred, prejudice and nonsense on a daily basis.
They then parade around as if their faith is something to be proud of and I am not allowed to speak freely out of fear for my life and job because I have chosen not to believe things without substantive cause. For example:
If you don't face this kind of bigotry on a daily basis then maybe you don't understand what others are going through, but I assure you it happens, frequently.
So, if someone seems too aggressive or outspoken on the subject maybe you should wonder what their reasons must be to have reached that point? I personally do not care what others choose to believe in private, but I do care about how they decide to practice it when it impacts my life.