My researcher treated me to a drink and icecream

Because I helped my senior researcher to carry out experiments for open campus voluntarily, he treated me to a drink and icecream.
Not only today but also in daily lives, he often treat me to a drink or bread. I've already known that it is the convention for the senior to treat the junior. So, when I'm enrolled in master's degree, I have to spend a lot of money on my junior researchers.
Of course, this expense is within my estimation. So, I make it a rule to refrain from being treated by my researchers in order not to be required to treat my junior researchers in the future.