Why would you reject something (i.e. God) that you find to be 'good'?
God and Satan are polar opposites. One such pair of opposites is God = love, Satan = hate. Love is good, hate is bad, no? Now if I have more hatred than love inside me, a question arises whether to try to fight it or embrace it. Many philosophies and religions recommend the former even though most of us feel a lot of hatred many times during our lives. Another possibility is to acknowledge and embrace hate. This doesn't mean that you have to act it out, though. Or you may.
I have decided to follow Satan which among other things means acknowledging hate and not fighting it. Thus I reject God.
But this is just one of many reasons.
I don't see God and Satan as polar opposites at all, even within the Abrahamic faiths. They are two separate things with their own unique meanings. Within Judaism Satan is either God's heavenly prosecutor, or is the associated with the words 'ha-satan' meaning 'the adversary'. In either case Satan is unable to act without God's permission. Nowhere is he in any sense a rival of or the opposite of God.
In Christianity and Islam, Satan was created by God as an angel who rebelled against God's demands, for various reasons depending on which faith and denomination you read. Again, Satan is not an opposite of God but rather an angel meant to serve and honor God. In both religions, Satan can be seen as the angel who would not 'bow' before Man when demanded to by God because either he (Satan/Iblis) was fully devoted to God alone and refused to bow to anyone else as in some Islamic sects, or that Satan as the Serpent in the Garden of Eden guiding Adam & Eve away from the Abrahamic god and His faiths which are the chains of spiritual oppression holding back Mankind from true liberation.
In Satanism, Satan takes on various functions. Theistic Satanism or spiritual Satanism is the umbrella term for religious beliefs that consider Satan as an objectively existing supernatural being or force worthy of supplication, with whom individuals may contact, convene and even praise, rather than him being just an archetype, symbol or idea as in LaVeyan Satanism.
The word 'Supplication' means the action of asking or begging for something earnestly or humbly", it deserves attention from the definition in that this is a key difference between Theistic and non-Theistic Satanism. Non-Theistic Satanism/Luciferianism has no supplication involved, does not believe in deities other than archetypal structures, or symbols.
For non-theistic Satanists, Satan comes to represent the symbol of a rebellious libertarian-style individuality and non-conformity. As a celebration rather than a condemnation of his humanity, a seemingly non-stop lust for imagination, exploration, and creativity.
As for Good & Evil, these are subjective terms as one man's good will be another man's evil.
To accept the idea that a God exists one would expect this God to embody pure goodness, and thus it would make sense to embrace this God. If you allow your Satanism to be defined by the Abrahamic faiths, then Satan is the embodiment of all that is evil in the world, and thus it would be a mistake to embrace Satan.
