Because if you hold down the sprint key, it'll try to sprint whenever you're moving forward and not doing an action anyway. It also seems an action will always interrupt sprinting, but sprinting will never interrupt an action. And on top of that it doesn't seem like aim is affected by sprinting, meaning there's basically no reason to not hold the sprint key down.

So the result of following this guide will just make sure the "sprint" key is also the "move forward" key and will save you from needing to push a separate sprint key ever again.

...Sprinting is also very fast in this game, so I also can't guarantee that auto-sprinting won't give you a headache. But still.

You can also blame the Risk of Rain 2 AutoSprint mod for making me want to figure this out but that's pretty irrelev-

It's surprisingly easy to do, and the trickiest thing will be making sense of the config file we'll be editing. All we will be doing is making sure the forward key and the sprint key are using the same binding.

• Be sure to change the sprint sensitivity to 1.0, and maybe consider turning off headbob too. Even without this guide, I would recommend this. (Headbob in particular makes it so when you sprint the FOV changes. Which I can promise you'll get sick of if you get auto-sprint to work.)
  
  
• Go in-game and make sure the control bindings are how you want them to be first. If you change these later, it might break the auto-sprint function. (In which case just go through this guide again.)

• You'll want to go to your save game location. On PC this is the following:
  %USERPROFILE%\Documents\My Games\Ziggurat2\
  
  (If you copy and paste that into Run or the Windows Explorer, it'll take you to where you need to be. Otherwise, it's going to be somewhere like "C:\Users\YOUR-NAME\Documents\My Games\Ziggurat2")
  
  
• Open the folder that is just numbers. (It's your Steam ID)
  
  
• Open the file "controller.json" into notepad. (Right click -> open with)

(For the purpose of this guide, make sure you are looking at the Keyboard sections of these next steps, and not the Gamepad sections.)

• The goal is to make Move Forward and Sprint the same key, and we're going to look at the "Move" bindings first. This might be different for you if you changed the movement keys, but at the top of this document for me is the following:
  
  
  {Inputs:[{id:0,name:"Move",type:"axis2",category:"",group:15,order:0,Keyboard:[{type:"axis2",idDown:115,idUp:119,idRight:100,idLeft:97},{type:"axis2",idDown:274,idUp:273,idRight:275,idLeft:276}],Gamepad:[{type:"axis2",id:0}],Mouse:[]}, (...continues...)
  
  We want to copy the number of "idUp:___", from the "Keyboard:" section of the code (119 is the "W" key.) This id number is the move forward binding. (I don't know what the one that says 273 is, I'm assuming it's the up arrow key.)
  
  
• Next we want to find the "Sprint" bindings part in the document:
  
  (...below Move, Aim, Jump, is Sprint...) {id:3,name:"Sprint",type:"button",category:"",group:2,order:3,Keyboard:[{type:"button",id:119}],Gamepad:[{type:"button",id:64}],Mouse:[]}, (...continues...)
  
  (I already changed my binding to match the "W" (119) key here, so for you this is most likely different.)
  
  Paste your "idUp:___" number (119 is the "W" key) into where it says Keyboard:[{type:"button",id:___}]
  
  If your move forward key is W, it should read as Keyboard:[{type:"button",id:119}]

If you've done it right, you'll sprint whenever you move forward. If you hate it, just change your binding for sprint in-game.

Again, be sure to change the sprint sensitivity to 1.0, and maybe consider turning off headbob too.