How can a person be unthankful to the one who does him/her good? How can a person be so unthankful to the One who showers him/her with His bounties and favors? how can s/he be so unthankful to God!
"Do they not look at the Camels, how they are made?- And at the Sky, how it is raised high?- And at the Mountains, how they are fixed firm?-And at the Earth, how it is spread out? Therefore do thou give admonition, for thou art one to admonish. Thou art not one to manage (men's) affairs. But if any turn away and reject Allah,- Allah will punish him with a mighty Punishment, For to Us will be their return; Then it will be for Us to call them to account." (The Quran: 088- 17: 26)
There is no heaven of glory bright, no hell where "sinners" roast. Here and now is the day of our torment! Here and now is the day of our joy! Here and now is our oppurtunity! Choose ye this day, this hour, for no redeemer livith! Say unto thine own heart "I am mine own redeemer!"
What's the purpose of life if there is no afterlife where the good will be rewarded and the evil punished. God is JUST and won't let the good deeds go without reward and the evil deeds go without punishment, therefore there is heaven and hell.