Selfish or just Sensitive?

Me and my current boyfriend generally bump heads on issues that seem to be less of an issue in my opinion but a huge issue in his. Our most recent disagreement was my timeliness. My boyfriend has had his car repossessed within the last year and has been loosing and gaining jobs quite a bit. Seeing as though he has no car, he asks me for a ride to and from work if needed. I had no problem with it at first because of course I didn't mind doing these type of things for him while he got back on his feet. One day I went to pick him up from work and I was running a bit late and ended up picking him up 10 minutes after he got off. Im not thinking its a big issue because it was only 10 minutes, but he feels like I should have been on time. This caused a huge argument all the way back home. He was completely upset!! I understand his reasoning but I don't understand why it had to be taken so far. I explained to him that I was late for whatever reason but that I felt that he was being unappreciative of me doing this for him because he could have easily caught the bus!! He gets even more upset because he feels like there was no reason I should have been late. Am I wrong in this situation for fighting so hard to defend my point?? Sometimes I see things more so in my favor (as in anyone) that Im blinded by what is right and wrong. Am I being insensitive to him for saying the things I said?? Just would greatly appreciate some insight.