Ease of Access and Implication for Design

You are more likely to consume something that's easily accessible and readily available regardless of need, or lack thereof, than to consume something less accessible/available. 

I once caught myself eating a snack (free snack that is) given out on a flight when I suddenly realized: first, I wasn't hungry at all, so why am I eating the snack in the first place? Second, I promised myself to not eat this particular snack again because I developed an upset stomach the last time I ate it. Inspite of the reasons I had to not consume this snack in this situation, I still did anyway. 

Most people have previous experience with a similar situation--consuming something with no real purpose, need, or thought behind it--not that one needs a purpose for each consumption or action. But why do people often act on impulse? Why do we gravitate towards the things that are easier to attain or do? Sometimes consciously, other times unconsciously.

People are situational. So are interactions (that require people). This is basic nature in human behavior- reacting to artifacts and situations presented in specic temporal instances in the world. People automatically respond to things that bring them perceived pleasure. The more accessible the artifact, and the less energy required to perform the action the more likely an action will occur. Ofcourse the concept is way more complicated than that.

But in summary, ease of access increases the likelihood of consumption or action.

And this is something that designers (in broad terms) can design for, or against. Whether it be increasing or curbing the consumption of food, entertainment, digital device usage, texting and driving, and etc. 

In conclusion, make it easy to do and readily available if you want it done more. And anything else, especially that which doesn't lead to meaningful and desirable actions, is a distraction away from the core purpose of your design.