I did some research on this topic several years ago, to address my own lateness problem. Most people assume it's because the person doesn't care, is disrespectful of others, or is self-absorbed. Those assumptions are incorrect. The truth is that habitual lateness is due to the person feeling like they have enough time to do "just one more thing". But then that one more thing or the task after that, takes longer than planned....before you know it, you're late!
I do agree with one of the responses here, in that lateness is a common symptom of AD/HD and other disorders like that. That is a wonderful reply and I was so grateful to see it, since I recently learned I too have AD/HD.
People who are habitually late may not show it on the outside, but they do care very much, so much that being late can cause severe chronic stress. They often will quit jobs, end relationships, etc. to help alleviate some of that stress. It is HORRIBLE to live that way!
You may think someone who is habitually late is just self-absorbed- consider that he/she is late because they are attempting to take care of everyone else's needs. Imagine this VERY common scenario: A young mother is trying to get herself ready for the company picnic. While getting dressed, she runs to the oven to take out the dessert she promised to bring. Fixing her hair, she remembers that her two young children need their faces washed again and another outfit, after the big mess they made with their snack at the table. While cleaning the table, she realizes the dog threw up on the floor and quickly cleans that up. She considers asking husband to help, since he is just istting there watching tv after needing only 5 minutes to get ready. Alas, it just seems easier to take care of it now while she has paper towels in hand. Her mother calls. Sister is having an emotional crisis and could she please go talk to her. Sorry mom, I'm running late gotta go. Now that everyone is ready, she has her husband get the kids in the car, while she puts on her shoes. Rushing out the door, she spins around to run in and get the dessert. While she's inside, she realizes that she forgot to use the bathroom and now it's urgent. Finally she dashes back out to the car and they're off. As the family drives away, husband checks the time and informs her they are 30 minutes late. She asks herself why it is always this way when she needs to be somewhere at a certain time.