Why is 一緒に correct and 一緒で incorrect?

Question

This is a mistake I habitually make, even though I know what the right form is. Earlier tonight, I wanted to tell my friend "Let's go together!", so I sent a mail saying: > 一緒{いっしょ}`で`行{い}こう！ My friend sent back a mail : > はい、一緒{いっしょ}`に`行{い}こう `;)` ... reminding me that `で` is incorrect, and `に` is the appropriate particle in this case. Which I know is right, but the reason it's stuck in my head to use `で` is that it makes more sense to me. `に` is used to indicate direction of action(?), so I don't see how "going" can be directed *at* "together". `で` makes sense to me, because my understanding of `で` is that it means "by use of" or "by way of" or something like that. So to me it makes perfect sense to say `一緒{いっしょ}で行{い}こう` because with `で` it means "let's go, and the way we'll go is together". Can someone help me break this habit by making some kind of logical sense of why `に` is right and `で` isn't?

Answer 1

Think of it this way:

"ni" is like indicative of direction, but "de" is mode of transportation.

Issho ni densha de ikou!

Let's go together by train!

Hope this helps!