Kakashi was a strict, by the book person that never questioned the rules. Because of
his strict following of the rules, Kakashi came off as a cold, heartless person.
The reason for this is because when he was little (6 or 7), his father committed suicide
because of his guilt and the harsh treatment he received from the village and his teammates after
he abandoned a crucial mission in order to save his teammates' lives. Kakashi didn't want
to end up like his father, Sakumo, so he started following the rules.
After the events of
Kakashi Gaiden, Kakashi would adopt a lot of Obito's traits, like being late and the bad
excuses. The real reason for his lateness is because he would visit the memorial stone
in honor of all those he lost and to remind himself of his past mistakes.