It's actually very simple.
God gave you earth to walk on, air to breath, ears to hear with and eyes to see with. He gave you the ability to choose to live your life in the manner you see fit, and even provides those who don't believe in Him with sustenance.
Let's say you're a good person to others; you help old ladies crossing the street, feeding the homeless and donating to the poor; maybe you even sponsor a child in Africa. These are all noble things regardless of what religion you belong to.
The key, however, is the intention. You are living your life not trying to thank God in any way, Who gave you all of this (including the sustenance which allowed you to be so generous). Because you were a good person, God gave you rewards in this life. Maybe because of your charity God helped you get a job you had wanted, or blessed you with another child. These are all signs, if you were to pay attention, of His Mercy and His Grace upon you.
But He does that all for you, even though you do not thank Him or praise Him (or even believe in Him) at all.
So how can you honestly expect God to give you a reward in the next life, when all you were doing is working towards your reward in this life? If you don't even believe in a Hereafter it makes even less sense. You live your whole life in denial of the Hereafter and you do nothing except focus on this worldly life.
When you come to realize you were wrong after you die, then why should God suddenly grant you entrance into Paradise when you had lived your entire life in denial of God and His Bounties?