Perhaps when thinking of rationality in economic terms, think of the most raw and basic thing you can. Assume there is an entity and assume it has a criteria for decision making. Voila! You now have something that makes rational decisions. This is because making a decision based on that criteria is rational where the other options (making them against the criteria or randomly) are not rational.

When this is translated into human consumer terms, the best economists have come up with is that people make choices based on wanting more benefit (regardless of what "benefit" even truly means).