There are only 3 times that I truly miss being part of a relationship. 
The first is obvious: Valentine's Day.  I miss being part of a partnership and relationship because this day is the day that most couples really show love for another in a public way.  I don't know why I miss it since none of the men I've ever dated celebrated the holiday at all, but I guess I miss the idea of what that day could represent.  I do love that I can show love for myself on that day and I do... I haven't decided what to give myself this year...
The second time I miss being in a relationship is when I want to share the joy with someone.  Sometimes when something good or funny happens, I just want to call my special someone and share it with them. I know, I know, I can share it with all the wonderful friends and family that I have been blessed with but somehow it's not the same.  Close, but not quite.
The third time I miss being in a relationship is days like today.  I woke up with the most excruciatingly painful kink in my neck/spine.  It's made it so I can't lift my head up, I can't turn to the side and nothing feels comfortable.  Days like this I wish that I had a significant other who'd want to take care of me, make me soup or rub my overly tense shoulders.  I miss that... I miss being cared for the most.