It can be very similar.
One big difference is usually we are with our SO’s on a daily basis, especially if we live with them. So the absence of them is felt much more than when time goes by not seeing or communicating with a friend.
If you are married, the commitment of marriage psychologically is different than a close friendship. Friends can drift apart, nothing bad might have happened. But spouses typically don’t just drift.
My husband knows everything In have been through in the last 20 years. He has gone through them with me, and I have been through his trials. The shared experiences unite you in a way that few friendships have.
An SO is somewhere between friendship and family, sort of a unique position. Spouses are generally the most important person in someone’s life, aside from children.
@Facade when you wrote SO, did you mean boy/girlfriend? Spouse? It is all a little different.