I work in an office downtown and the other day I forgot lunch. I had class that evening till 10, so I knew I had to eat something. I was extremely busy so ordered a sandwich delivered. It ended up costing like $12, which was around $7 for the food and $5 delivery fee. Under that it said the delivery fee all goes to the delivery guy. It recommended a tip but I decided not to leave one. 

Well, when I picked up my food from the front desk, the delivery guy started asking if I wanted to leave a tip. I said "no" and he asked if I was 100% sure I absolutely did not want to do that. I got a little uncomfortable but said "no" again and left. He scowled at me and left. The receptionist looked at me sideways and said bye to the delivery dude. 

I feel terrible for not leaving a tip. Am I the asshole in this scenario. 

TL;DR: did not leave a tip for a delivery guy and he was a little salty. 