This sticking point arises primarily from what Christians believe God to be – and most Christians believe God to be OmniGod or Supergod -- an omnipotent, omniscient, omnibenevolent Creator who created everything from nothing and is the originator of everything but Himself.
Within this conceptualization, man is a creature whose capacity for creation is limited to subcreation – more specifically, the rearrangement and modification of God’s Creation. Man and all of God’s creatures are limited to subcreation by the simple fact that everything in Creation is of God. Unlike God, man is incapable of creating something out of nothing, which entails that man – and all other creatures – can never really create anything original or add to Creation in any way beyond the limits of modification and rearrangement.
This understanding of Creation is heavily steeped in Classical rationalism and Medieval Scholasticism, and Christians who adhere to this understanding of Creation declare it as evidence of God’s unquestionable glory, power, and supreme reign over Creation – a supreme reign that does not require nor desire co-creation – not only because it is impossible, but because it would be superfluous.
Within this conceptualization of Creation, God neither yearns for nor desires any real creativity or origination from his creatures because such creativity or origination would pose a direct challenge to his own status as prime and sole creator.
The idea here is simple – original creative activity is limited to God and God alone. If it were not, God would not truly be God, but something less-than-God.
Any sharing or expansion of original creative activity – were it possible – would not only shake the very foundations of Creation, but would also immediately diminish the stature of God.
Thus, instead of a God who actively yearns to share original creativity, rational theology arrives at a conceptualization of a God who contains all original creativity within Himself and is, ergo, perfectly content to engage in what amounts to a game with Himself.
However, if the conceptualization of God as OmniGod who created everything from nothing is set aside, it becomes possible to conceive of Creation as constantly evolving, developing, and expanding rather than as something complete. Within this continuously evolving, developing, and expanding Creation, co-creation not only becomes possible, but desirable as the ultimate purpose of divine Creation, which becomes voluntaristic rather than intellectualistic in nature.
Rather than being an originator of everything from nothing, God becomes the primary creator of a Creation consisting of pre-existing beings that voluntarily agree to be a part of His Creation so that they can participate in the opportunity to align with God’s will by choosing good over evil, creation over destruction, and, ultimately, divinity over death.
A big part of choosing divinity over death entails the motivation to become divine, which implies that God does not view divinity as something static, as something only He is capable of possessing, but as something that He can share with and expand to others. The motivation to become divine lies in the essence of co-creativity, which is a higher spiritual achievement than subcreation. Co-creativity increases the divine within man without the expense of any diminished divinity in God.
On the contrary, any increase in the divinity of man adds to and ennobles the divinity of God because man can only approach divinity by freely aligning himself with God’s Will. Rather than elevate man at the expense of God, co-creativity – as a divine-human operation – elevates both man and God.
Co-creativity not only represents a higher spiritual pursuit than subcreation, but it is also a higher expression of freedom and love. God proves his love for man by freely offering and sharing the potential for original creativity within Creation, which is based in freedom and love.
Man proves his love for God by answering God's call and helping to add something original to Creation that God could not have added alone. This not only adds to evolution and expansion of Creation, but it also adds to evolution and expansion of divinity via the evolution and expansion of freedom and love.
Note: Freedom presents difficulties to the Classical rationalistic/Medieval Scholastic definitions of God as an omni-everything. If God created freedom and freedom is wholly within God, then the freedom we experience is not really freedom, but a sort of partial freedom or psuedo-freedom. One possible solution to this dilemma is to posit that freedom is uncreated. If freedom is uncreated, it is outside God, which raises the possibility that God does not and cannot control freedom. If this is the case, the Classical rationalist/Medieval Scholastic conceptualization of God as omni-everything becomes riddled with inconsistencies. Freedom as the primordial foundation of being is another idea I've come across. This idea appears mostly in sources written by Christian mystics.