Why is it so hard for people to accept God into their lives ? To be honest with you I really do not know why, but here is what I think about the whole thing. I think that people just want to live out in the world and do whatever they want to do. They think that God does not exist and really and truly he does.
I think you have suggested a big part of the answer. Let me put forward further thoughts for you to respond to.
True, some people do not want God interfering in their self-centred lives and some refuse to believe God exists; they might feel accountable to Him if they believed there was a God.
However, many do believe in God, but still leave Him outside of their lives because they wrongly think they can avoid Hell and get to Heaven by their own efforts or good living. Yet others vainly choose religious ritual or observances as a way to try to please God without actually accepting God into their lives. They still want to retain control of their lives.
Some don't believe in Hell, or think God would not, in the end, consign anyone there. He would just let then into Heaven, effectively lowering His entrance standards. Others wrongly feel they are too bad, and God will not deal with them graciously. Others have "loved darkness rather than light, because their deeds were evil". Such a person "hateth the light, neither cometh to the light, lest his deeds should be reproved."
Spiritual blindness is surely the biggest culprit, as the Devil, Satan being "the god of this world hath blinded the minds of them which believe not".
Do you have any further thoughts or questions?