system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do we get blisters when wearing new shoes for the first time?
gpt
The average foot is covered with fairly soft and scrapeable skin. That skin wears away when it's rubbed or tugged against something over and over again. If the constant pulling action is along the surface of the skin and it's in short quick jerks, it can actually separate the top layer of your skin from the bottom. Fluid fills the middle and you get a blister. 

So take that foot and jam it into a shoe that doesn't fit properly and you create a number of pinch points. As you walk and your weight shifts that foot around a little in your shoe over and over again in the same way, the conditions near the shoe's pinch-points can be perfect for this tugging action to occur, resulting in blisters.

If the shoe is a "near fit" and you continue to walk in it over time, it softens due to the repeated stretching, some of the seams wear down a tiny bit from friction, and its leather or other content molds and stretches to accommodate your foot better. This reduces the pulling action because it's no longer pressing against your skin in the same way, so you don't get the blisters any more.