We all have a sin nature, but understanding why we sin isn't that complicated. Typically there are three (3) basic motives for sin:
1. Committed unintentionally.
We did it without realizing it was a sin or without realizing that we did it.
2. Committed out of weakness or negligence.
We did it because we were not focused on doing right, and just drifted by default to wrong.
3. Committed out of rebellion.
We knew it was wrong when we did it, but just didn't care.
Of course, none of these are reasons any of us would be proud of. So if I say that I "struggle with" some sin or sin pattern in my life, then I really just have to ask myself which of these three underlying explanations is applicable to that struggle.