It's not irrational. Is your goal wealth maximization? If so, you might understand that you're now getting an extra $50/mo for food purposes, but now you feel

It's not irrational. Is your goal wealth maximization? If so, you might understand that you're now getting an extra $50/mo for food purposes, but now you feel like you should hold it and not spend it. Is your goal to suck down some minor pleasure to keep your brain mollified as you ponder some unsolved task? Well maybe the distraction posed by thinking of soda; thinking of buying a soda; thinking of buying a soda and wondering if you have the right change for it; thinking of buying a soda and wondering if you should really save that money to spend on a larger purchase later; etc... Anyway, maybe that distraction is greater than the distraction posed by grabbing a free soda. People's different reaction to free vs non-free things, especially in the context of engineering work, does not strike me as irrational in general—only if you think that people maximize for present dollars does it seem particularly irrational.