In a desperate attempt to refute Christianity, many atheists like to come up with reasons they think people are Christians other than evidence. This is called the genetic fallacy, i.e. criticising a belief based on one's perception of its origins. Any argument that begins with "you are a Christian because..." fails as an argument against Christianity because people's reasons for believing a thing does not make that thing factually incorrect.
As an analogy, I really hate the smell of cigarettes. Would it be valid to say to me "you only believe smoking increases your risk of lung cancer because you hate the smell" as an argument in favour of smoking? Of course not! That would be a ridiculously fallacious argument that no one sensible would even think to make. Similarly, even if it was true that my sole motivation for being a Christian is the desire for eternal life, that has absolutely no bearing on the accuracy of Christianity.
But aside from being a fallacy (and aside from being flat out wrong in my case), it doesn't even have any explanatory power. It may be true that I want eternal life, but Christianity isn't the only religion that promises it. It's the only religion that can follow through on that promise, but it's not the only religion that makes that promise. Other religions, such as Islam, also offer eternal life to believers. So why am I a Christian and not a Muslim?
In fact, as awesome as eternal life sounds, Mormonism goes as far as to promise Godhood. Good Mormon men believe that good Mormon men will one day become gods, and rule over their own planet, which they will populate with their own spirit children. And even better, those spirit children are born to their spirit wives, whom they married on the earth.
To me, personally, this is a much more attractive option. There are aspects of Christianity that I don't like, chief among which being that marriage ends at death. I do not believe I will be married to my wife in Heaven, and that is a particularly unattractive view to me. But Mormonism disagrees. So why would I be a Christian, which contains an unattractive doctrine, and not a Mormon, which changes that doctrine in a way I would prefer? Eternal life, Godhood, eternal marriage, if I was going to choose a religion based on desire alone, I'd be a Mormon.
But I'm not a Mormon. I'm not a Mormon because I don't choose my beliefs based on desire alone. I believe what I reason to be true. Christianity may be attractive because it offers eternal life, but it is attractive to me because it is a credible offer of eternal life. Christianity is true.
Coincidentally, the method by which Christianity offers eternal life also happens to be the method by which God proves Christianity to be true. Eternal life is offered to us through the death and resurrection of the Lord Jesus Christ, and it is accepted by the confession of His Lordship and the heartfelt belief in His resurrection. And of course, through His resurrection, Jesus proved His power over death. We can know that we have eternal life not because we merely want it, but because Jesus, having been publicly executed, got up and walked out of His tomb. Eternal life is an attractive offer, eternal life is a credible offer, eternal life is a free offer. I just can't see a better alternative.