There can be several factors for it, as mentioned by some of the previous posters.
1) Stress. I feel that can be a prime factor. It tends to block out the needs of others.
2) Upbringing. Some good habits are learned or bad habits are learned from the home environment while growing up.
3) Ignorance. I'm sure that I unknowingly cut someone off while driving. Sometimes we all zone out and are not aware of our surroundings.
But we do need to try and look for the good in others or give them the benefit of the doubt.
Alligator Wisdom (aka Brother NOT Exerting Vigorously by WTS standards)