What a wonderful & thoughtful surprise for your breakfast. You know, you could pay it forward.....sometime in the future. However, this was something that made the young man feel good.....so you should just accept it. I like to think the person perhaps lost someone, mother, dear friend, and seeing you had him thinking he wished that person was there, so.......
Several years ago, I was driving to Chicago, a friend here wanted to visit her daughter in the Racine area, so drove with me, meeting the daughter at the oasis where the the interstate divided, with one part off too the left to the Milwaukee area, and the other continuing to Illinois. Well......we stopped for lunch in the Dells area, and as we got in line at the cashier, the woman in front of us, turned and said she was paying for our bill! My friend was adamantly refusing, but I told her we should let the woman do it, she has a reason, and we cannot spoil it for her. My friend backed down, but insisted then on putting the money into a charity cup, (a different spot in the restaurant).
When someone does a kind thing.....one should be gracious and accept it. Not doing so spoils it for the gift giver, who is feeling good because they are doing something nice, & unexpected for a complete stranger.
Guess we all may look at the situation differently.