You prefer a smaller reward now over a larger one later.

A 1975 study found that the subjective value of a reward decreases hyperbolically as the delay to receive it increases.

Hyperbolic discounting describes how you devalue future rewards as they get further away in time. You are not irrational; your brain is simply wired to favor immediate survival. This explains why you struggle to save money or diet: the 'now' is consistently more powerful than the 'later'.