If there is a God, then without him, you do not exist. Do you want to be that alone?
Nope!
Of course if there is a G-d, perhaps if you believe in him, he would make you alone.
Perhaps you like math. Perhaps the thing you like in math most is solving differential equations. For the first 10,000 years of your afterlife, you happily solve differential equations. After that, you have solved all the differential equations that there are. You really need to go on to something else.
No problem, the second best thing for you to do is to solve intergral equations. You quite happily solve integral equations for another 10,000 years. Alas, after 10,000 years of solving integral equations, you are also done. You have solved them all, and need to go on to something else.
So you are now reduced to performing differentiation, your third favorite thing. The after life is still good, but after only 20,000, you after life quality is significantly diminished.
And as time goes on, you see that you will be reduced to doing things that you are increasingly less enthusiastic about doing. Eventually you know that you will be forced to attend poetry readings, or engaging in group therapy with one of your ex-wives... Yuck.
Better to be a Buddhist, and hope for total annialiation.