As a Christian, I not only believe that I have been commanded to spread the Gospel, but I believe succeeding in this task will save people's eternal souls. It makes perfect sense for me to preach the Gospel. I believe it has eternal consequences. Other religions have similar motivations. They may be incorrect, but one can at least understand why a Theist would attempt to persuade you to believe in their gods.
But what about atheists? Atheists are very passionate about their beliefs. They won't just argue for their beliefs, but often they will do so with such emotion, it's obvious that there is a spiritual issue here. Why? Because if atheism is correct, it would not be so important that it would be worth preaching it.
If there is no God to whom we are accountable, why bother preaching about it? If atheism is true, your beliefs do not matter in the grand scheme of things. You are a clump of cells on a wet, mouldy speck of dust floating around a tiny burning ball of gas in a giant vacuum, most of which is completely indifferent to your existence. And you are guaranteed to die. What will happen after that? You won't know. You will have no memory of your past, you will have no knowledge of your future, you will have no perception of your present. And this is true whether you die as an atheist, a Christian, a Hindu, a Buddhist, a Muslim, a Catholic, a Mormon, if atheism is true, your beliefs have no eternal consequences what so ever. If you convince a Theist to change their religion, you have saved no one, you have not given them any kind of gift, you have not altered their eternal fate. They will rot in the ground, same as everyone.
And so why do atheists care who is and isn't an atheist? Why are they so passionate about their atheism that they will actually cuss a Christian out just for posting or commenting on a meme? You would think they would at least be going out trying to enjoy the one life they believe they have rather than getting angry with people who believe they will have another.
But atheism is not rational, and so it should not surprise us that its followers are so irrational as to try to proselytise even knowing it is completely pointless. And it isn't just a case of misery loving company. It's a case of suppressing the truth in unrighteousness. Man is sinful by nature. We rebel against God in many weird and wonderful ways. Atheism is just one of them. But like all the others, atheism is self refuting. Every time an atheist rages against God, they tacitly admit they have an oblique awareness that He is there. And He is.
There will come a time when all atheists will worship God. No one can run from Him forever. The question is how they will worship. For those who die in rebellion, the answer is unfortunately by force. But God has extended an alternative offer to mankind: Confess with your mouth that Jesus is Lord, and believe in your heart that God raised Him from the dead. Then you will be saved.