Why do we sin knowingly?
Is it because
* we have faith in Allah's (swt ) mercy (if yes, then is our faith stronger than our love for Allah?)
* we are weak against our nafs
* we think we are going to hell any way before reaching heaven
*If it's a mix of the above then which one is a stronger motivating factor
*other
I'm trying to understand why we commit sins even when we are aware that it is a sin.
Hm Good question.
Answer can be varied for everyone. And can be more than one factors. But I would go with the 2nd one in your list. We are very weak in the control over wishes and limiting ourselves. Otherwise its not that we dont know where are the boundaries. And we somewhat do curtail ourselves but still not as per the requirement. May ALLAH forgive us and give us hidayah
Weak impulse control and weak faith (if any).
In my opinion weakness is the initial or root cause. We are created weak. In terms of which is the strongest motivation, it can be an amalgamation of all 3 even. For example, a person can even use weakness as justification by saying that Allah created us weak and we are not perfect and this sin is my particular weakness. From there the person may move on to the justification that Allah will forgive us because He is very merciful and also because He knows that He has created us with weaknesses. From there, the person may also justify it by saying that it's pointless to become so consumed by the outcome of my current sin when I have committed many sins in the past that have already landed me a dwelling place in hell. I don't recall that I have ever heard that perfection is beloved to Allah and He has made us imperfect in our nature. But I have heard that repentance is beloved to Him and that gives us hope provided that we don't misuse this. Why has Allah called human beings the Best of Creation when the angels are sinless? The angles don't have to strive or fight against any weakness. We, on the other hand, have to strive against our impulses to avoid sins and to do good. A craft that is produced by your own hands, your own hard labor, will be worth more and will cost more than something that is churned out in a matter of seconds from programmed machines in a factory.
I believe that it is beneficial for us to identify what motivated us to commit a sin so that we can try change our ways. But at the same time, thankfully we have a Rabb that does not shun us after we have erred the way that we do with each other. Instead He wants us to return to Him even after we have erred. So, there is hope.
Well let us not forget Iblis! 
is always on the Job telling us it will be just fine 
Its the Shaitaan. He can influence our nafs. When he had disobeyed Allah he vowed to mislead us from all directions in any way he could. Looking at man's history he has been largely successful in this. And it is because of Allah's Mercy that we will be saved from hellfire. Allah had said to Shaitaan that as long as my servants ask for My forgiveness I will continue to forgive him. The operative word here is continue.
If and when we think, we are going to hell anyway, so might as well commit sins, that is also Shaitaan with his whispering tell us and misleading us. He will continue to attack us so that we keep committing some kind of sin. His ultimate goal is to have us do shirk.
Keep your tongue moist with the following adhkaars (along with your daily salah):
Allahumma ajarni minan naar --- The hellfire will ask Allah to keep you safe from it.
Allahumma adkhilkni jannat ul firdos -- Jannah will ask Allah to admit you to it.
Peace LP,
It is human nature either to fall in error and guilt or commit sin as it is narrated in Hadith: "It was narrated from Anas that the Messenger of Allah (ﷺ) said: ?Every son of Adam commits sin, and the best of those who commit sin are those who repent.??. As hadith says, everyone will commit sin either knowingly or unknowingly but the best who repent.
But why we are best after repent? Answer of this is in Hadith which Narrated by Anas bin Malik: Allah's Apostle said, "Allah is more pleased with the repentance of His slave than anyone of you is pleased with finding his camel which he had lost in the desert". Means, that feelings when we found a lost thing which is more dearest to us and we keep it with more extra care so that we will not lost it again. So when we commit sin, we are kind of lost like 'thing' and when we repent, we return back to Almighty Allah. Almighty Allah loves those who repent. Here is another way Almighty Allah wants us (His creations) to understand/realize that we shouldn't despair from His mercy no matter how often we commit sins. Still Almighty Allah loves us (being His creation) when we repents.
Almighty Allah says in Qur'an Chapter 2 Verse 222:".......For Allah loves those who turn to Him constantly (constantly repentant*) and He loves those who keep themselves pure and clean*".
Below hadith describe love and mercy of Almighty Allah to His creations:
Abu Mu'sa reported Allah's Messenger (may peace be upon him) as saying that Allah, the Exalted and Glorious, Stretches out His Hand during the night so that the people repent for the fault committed from dawn till dusk and He stretches out His Hand daring the day so that the people may reprint for the fault committed from dusk to dawn. (He would accept repentance) before the sun rises in the west (before the Day of Resurrection). A hadith like this has been narrated on the authority of Shu'ba with the same chain of transmitters.
What I understood that it is a way to get closer to Almighty Allah and to become among those whom are loved by Him. So we refrain from committing sin because we love Almighty Allah and we have faith in Him that if we repent, He will forgive us. Almighty Allah created us weak that is why we commit sin, either knowingly or unknowingly.
You will always be fond of me, I represent to you all the sins you never had the courage to commit.
We simply follow sunnah of Adam and Eve and nothing else! We were born to sin!
A man's ethical behaviour should effectively be based on sympathy, education, and social ties and needs, no religious basis is necessary, Man would indeed be in a poor way if he had to be restrained by fear of punishment and hope of reward after death.
Two of the biggest factors for committing or not committing sins are fear/no fear of punishment and hope of/no hope of reward after death, so who is going off on a tangent? the comment was totally relevant to your question.
The thought process is many people are not afraid of punishment, nor do they hope for any reward after death, so commit acts which may be perceived as 'sins' by those with the opposing views. A person would not consider an act a sin if for them it does not breach their definition of 'ethical behaviour' which i pointed out above, for example drinking alcohol, engaging in sex outside wedlock, engaging in homosexuality etc etc On the other hands those believing in a religious and so called divine direction of what is permissible and what is not will call those acts 'sins'.
In a nutshell, many people will commit acts which may be 'sins' for you but not for them. They are more bothered about the ethical behaviour of social needs, education, sympathy etc. Hope this helps.
or unless the 'We' you mentioned is not something inclusive, ok my mistake!
Understood ^
If you allow I could attempt to answer with my religious hat on, will you?