I have been playing mostly solo for the last 10 years. I will also second Jack’s points but add/extend one as well. I have not found it necessary to have an ongoing campaign following a unit is needed to be interesting (although Jack does mention to me that I *must* do so. He may get his wish soon as I am probably soon going to do a 5 Parsecs From Home campaign). But I do believe something equivalent to this is important. I found that for the first four years, it was replaying the same battles with different rules that kept me interested in solo playing – uncovering how different rules worked was fascinating. I then moved onto playtesting my own rules. So that was (and still is) interesting in seeing how they play and tweaking them after every game 🙂 I have also done a series of related games, so not quite a campaign with the same unit but something like the Operation Jupiter scenarios, or a sequence of fights centred on a particular operation. So not a campaign with the same unit, but something similar just the same that helps maintain the interest.
And you do have gaming friends Jack, they just live thousands of miles away from you. No idea why 🙂