Because at birth everyone is apparently cursed to do so.
And only if you stay true to the Bible may you walk the path of righteousness.
When I was younger I always thought sin was like the hiccups; You can't really fight it and they just show up and leave on their own accord.
So when I was younger I simply gave up fighting it. No use fighting something that can't be stopped right?
And that's when the revelation hit me that I was just fighting myself and what naturally happened. And that's when I fell.
You can't stop sin. Thinking aggressive thoughts about someone is a sin, jealousy is a sin, lying, even unknowingly, is a sin.
You can't escape sin, so of course you have to apologise and repent.
It says non-Christian by your name, but you pointed out a few things that even those calling themselves Christian would disagree on, which makes the subject of "sin" kind of comical, but also very telling.
Treads about sin always seem to become one of those "opinion" topics that some will say is only based on how people "view" and "interpret" the bible. However that's not the case, because the bible clearly describes sin; what it is and what it is not.
Why so many opinions then? Because some are saved from God and some are not. In any case your statement, "Because at birth everyone is apparently cursed to do so." is spot on to what the bible says of man. Man is born into a sin nature and sin is that which is absent of God. However, people view it differently, only Christianity views sin as a state of being.
In Judaism sin is the violation of any of the 613 commandments. Judaism teaches that sin is an act, and not a state of being, but agrees that man's inclination is to sin.
Islam views sin as as anything that goes against the commands of God (Allah). Islam teaches that sin is an act and not a state of being.
Buddhism and Hinduism do not recognize the idea behind sin, but believe in the principle of karma, whereby suffering is the inevitable consequence of greed, anger, and delusion (known as the Three poisons).
In the Bahali Faith, humans are considered naturally good (perfect), fundamentally spiritual beings. Human beings were created because of the Bahali god's immeasurable love. However, the Bahali teachings compare the human heart to a mirror, which, if turned away from the light of the sun (i.e. the Bahali god), is incapable of receiving the Bahali god's love. This last part is similar to Christianity in terms of turning away from God.
Christianity is unique in that it is the only "religion" that views sin as a state of being, where man is born into a sin nature. However, as a religion it's not fair to say Christians on whole actually believes this. Just look at what's been expressed in this OP so far.
Some people calling themselves Christian, view sin as a matter of perception; a very specific action, or thought, and in many ways see it the same as Islam or Judaism; just a violation against God, rather than a state of being. Others calling themselves Christian have a mixed view and might say we are born sinless and only sin when we come to the age of accountability. Still others, like this OP, will develop their own unique view and freely contradict their own statements to the point where it is unclear what they believe.
In any case, any Christian view of sin should harmonize with what the bible says, and the bible clearly says that sin is a state of being for man since the fall of man, which is described as original sin.
The bible describes Adam and Eve, not only disobeying God, but doing so in a manner that changed their very nature, and that of all mankind placing enmity between God and man. This single event damaged, and completely severed, the relationship of humanity to God. That relationship can only be restored through repentance and acceptance of Jesus Christ and his death on the cross as a substitutional sacrifice for mankind's sin. Christ who is God offers this in Love for man who can not pay the price himself. This is where the church splits on many points regarding sin, the nature of man and the saving grace of God.
It's a pointless debate due to the nature of the saved and the unsaved. In any case if man is in a state of sin by nature, then he is also helpless due to his nature, just as a blind man is helpless to see despite any desire he may have to see, and therefore requires a greater intervention not of himself to change what he can not; unless of corse one wants to suggest a blind man is simply blind when his eyes are closed. If that where the case he could simply open his own eye at his own will, but that's not the case.
Again the bible says our nature is sin. In this way, sin is not just a conscious act, but a state of being, and in this way sin is part of who we are, and requires a new nature. So the answer to the question; Why Do We Sin, is very simple. Because we are sin.
This is an unacceptable answer to many who would rather A. believe they can choose at will to not be sin, or B. Believe by nature they are good, reasonably so.
Both of these lines of thinking belong to the unsaved (period) and the unsaved simply do not know the new nature and saving grace of Christ. They can not speak of what Christ has done for them because they have yet to experience it, and are either willfully trying to be like Christ, or could care less about Christ. Same people, same boat even though one group says they love Jesus, or are members of one of the other "religions" mentioned.
The saved would not say A or B. The saved understand they are sinners. The saved are convicted by the word because they have been chosen by God to be so, and have accepted the grace and mercy of Christ unconditionally. By this, the saved have been given a new nature, not a nature that allows them to NOT SIN, but that allows them to yield to the same Love, Grace and Mercy that converted them in the first place. The save know the difference, the unsaved do not, and in this way salvation is not anything that any man does unto himself, but is done unto him by God and God alone. Can we use scripture to prove this? Sure, but it still won't matter to those who are not destined to salvation. They will still see it as a perception alone.
Why do we sin? Because we are sin. Why are we saved? because we can't save ourself. Are we saved from sin? NO, we are saved from God because of our sin. Why? Because God chooses to do so. Why? Because God is love.
Although he may not agree totally on some of the non-essential matters of salvation I have mentioned, this is why Adullam responded with this.....
That is until you have a personal encounter with Christ.