For me: SO (and if they’re not the same: spouse) is more about becoming a team.

With a really good friend, unless you have something very special, you’ll probably not tether your lives together and commit to going through the rest of your days together.

There’s something that happens over time when you have that commitment that really bonds you (so I’ve found). I haven’t felt it with a friend, and the closest I’ve come to it otherwise is with family (though there’s even less commitment between family members, I’ve found).

It’s hard, but it’s also great.