The following consist some of the reasons:
-You don't think there is suitable evidence for a deity
-You actively dislike the idea of deities
-You haven't herd of or care about deities
-Religious reasons [no seriously there are atheistic religions like buddhism]
Does atheism necessitate the belief that reality is depressing and pointless? Nope.
Is lying to people to save their feelings a good practice? No, probably not especially because different sets of lies compete with one another or even compete with real facts, a mantra of ignorance further denies a person's choice of beliefs and the freedom of information.
Why do some atheists want to persuade others that there are no gods?
-Epistemological reason- just because an idea is nice doesn't mean it is true, defencible or special
-Some people pressure atheists to share their belief in a certain god, and debate is triggered as defence
-Some religionists' behavior, in the name of their gods, appears mad to the non religious- such as holy wars and ceremonial mutilation of children [includes male and female circumcision, facial tattooing, involuntary body piercing, denying a child medicine, removing teeth etc]
-The defence of real and established facts which are contrary to religious dogma.
TLDR This is your mortality, it is a fact of life, deal with it