The simple answer is because we CAN! Man has FREE WILL! My observation is that it starts awfully early in life. Babies are more prone to saying NO, before they learn to say the word YES. I’ve not done any scientific studies on that; however, my experience has shown me that to be true. Freedom is self-positing! As Norman Pittenger put it, “To be able to explain why I freely choose this or that, would be to say that I did not freely choose it.” Think about it.
In retrospect, or with 20-20 hindsight, we can usually discover the causative agents that went in to the decision making process. And when we get down to it, there really is no pure freedom! The whole concept of choice is bound up in the environment from whence we’ve lived for however many years. Face it- we’ve been made, molded, and managed by those to whom we’ve allowed into our lives. You may think you’re free, but you’re just acting out what “past living” has shown you.
It’s much like asking a fish to define water!
Define water? Since the fish has always lived in water, and only known water (as compared to land), there’s no way to define H2O. Did you hear what was not said there? The only way to be able to comprehend or understand GOOD is to have encountered a little BAD! Note the word “little“. A little bad can go a long way! 
For me, the best answer I can give, in light of where I’ve come from, is that we say NO to God because we really don’t KNOW Him. The more I have come to know Him (and that in itself is a choice), the more I’ve wanted to give my life to Him. The “giving” is more than an acknowledgment that He’s smarter than I am. It’s more than a public dedication or surrender. And, it’s more than some intellectual assent to a code of ethics or moral code.
Religion is really big on those last two!
When you see God as Marcello Craveri does, as he said, “God is that all powerful being who is tyrannically disposing of the destinies of all things”, you end up fearing God and in so doing, you run from Him. And, NO becomes the easy thing to do, hoping you’ll escape His wrath, and not be found out. I mean, how can God not know, or lose sight of you?
Just say YES, I want to KNOW you more, and the “NO’s” will slowly fade away. And so will your views on HELL (CLICK HERE).
Yes- for Jesus.