This is not true: "According to atheists, your life is all you have so you should take it seriously."
An atheist is not someone who believes that your life is all you have so you should take it seriously. Specific atheists might believe that, but one needn't believe that in order to be an atheist.
An atheist is someone who doesn't believe in gods.
That is all an atheist is.
Many atheists don't believe in an afterlife. But an atheist COULD believe in an afterlife and still be an atheist. Because an atheist ISN'T someone who doesn't believe in an afterlife. An atheist is someone who doesn't believe in gods.
That said, I am an atheist who doesn't believe in an afterlife. But this question is STILL not relevant to me, because I don't believe "you should take life seriously." You are welcome to take life seriously if you want to; if you don't want to, don't.
I see no reason why one SHOULD take life seriously. Personally, I really wish I could take it less seriously. Taking it seriously tends to raise my blood pressure.
Finally, DOES Christianity keep people from committing suicide? How do you know this? Do you have statistics to prove it? You'd need to somehow show that, say, 100 atheists and 100 Christians had contemplated suicide but more of the atheists had gone through it than the Christians. Can you do that? Just because Christian dogma posits a rule, that doesn't necessarily mean Christians will follow it. Christianity has a rule against murder, but plenty of Christians kill people. (As do plenty of atheists.)
Also, not ALL Christians believe the same thing, regardless of what it says in scripture. I have Christian friends who don't believe that people who kill themselves are damned. I guess you can say that they're not true Christians if you want. But what do you need to believe in order to be a true Christian? Do you need to believe everything in the Bible in a literal way? Are Fundamentalists the only real Christians? Atheists are also not all the same. IF we take it as a given that Christianity keeps people from committing suicide and IF we take it as a given that a subset of atheists (a) believes this to be the case and (b) thinks about the ramifications of it, then presumably -- since they are all unique people -- some of them are happy about it and some aren't.
I will say this: if someone proves to me that Christianity is doing something I think of as good, I am definitely happy about it. I am not in any way anti-Christian, anti-religious or anti-theists. Some atheists are, but a lot of us aren't. Because an atheist isn't someone who hates Christians (even if some atheists do). An atheist is someone who doesn't believe in gods. I don't believe in gods.
If Christianity causes something bad to happen, I don't like it.
If Christianity causes something good to happen, I like it. I don't like the Crusades.
I like St. Paul's Cathedral.