We all know the story of Iblis and how he became the Shaitan, Allah says in the Quran:

''And (remember) when We said to the angels: 'Prostrate yourselves unto Adam.' So they prostrated themselves except 'Iblis' (Satan). He was one of the jinn; he disobeyed the command of his Lord" (Quran, Surah Al-Kahf: 50).

"....And they prostrated except Iblis (Satan), he refused and was proud and was one of the disbelievers (disobedient to Allah)" (Quran, Surah Al-Baqarah: 34).

(Allah) said: "O Iblees (Satan)! What is your reason for not being among the prostrators?"(Iblees (Satan)) said: "I am not the one to prostrate myself to a human being, whom You created from sounding clay of altered black smooth mud" (Quran, :32 – 33).

"I am better than him (Adam), You created me from fire, and him You created from clay" (Quran, : 12). 

Self-pride has existed for a long time, we (myself included) think we're better than anyone else and that we shouldn't treat them with respect.

Al Bukhari and Muslim reported that  the Prophet Muhammad SAW said: 

"Allah will not look on the Day of Judgment at him who drags his robe (behind him) out of pride."  

Allah replied back to Iblis saying:

(Allah) said: "Then, get out from here, for verily, you are Rajeem (an outcast or a cursed one)." (Tafseer At-Tabaree).

"And verily, the curse shall be upon you till the Day of Recompense (i.e. the Day of Resurrection)" (Quran, Surah Surah al-Hijr: 34 – 35).

We see here that even though Iblis was disrespecting Allah, He still managed to grant him what he asked for.

(Iblees (Satan)) said: "O my Lord! Give me then respite till the Day they (the dead) will be resurrected."Allah said: "Then, verily, you are of those reprieved,"Till the Day of the time appointed" (Quran, Surah al-Hijr: 36 – 38). (Iblees (Satan)) said: "O my Lord! Because you misled me, I shall indeed adorn the path of error for them (mankind) on the earth, and I shall mislead them all."Except Your chosen, (guided) slaves among them" (Quran, Surah al-Hijr: 39 – 40).

The reason why we sin is that Allah has ordained the nature of mankind to be fallible. We are constantly facing hardships in life and wanting desires of this worldly life. Whilst we have the ability to steer our "free will" in any direction, we remain exposed to whispers that can stray us from the straight path.