I recently moved to another state, and while I miss my regular playing partners of old, I am also one of those who plays better as a single. First off, I hate being pushed by the group behind which can sometimes happen if you are part of a threesome or foursome. In that case, I find myself rushing my shot and screwing up.
Playing solo, I almost never get pushed by a twosome or threesome behind me, so that gives me plenty of time to concentrate over the ball without worrying if I am holding anyone up. Allowing a group to play through me is never an issue.
If I end up pushing a group ahead of me and they invite me to play through, I will almost always do so. But I would never ask for the privilege unless they were painfully slow (i.e. a group of obvious beginners).
In the same vein, I would never ask to join an existing twosome or threesome. I would think that would be presumptuous on my part. If they should
invite me
to join
them
, however, I may or may not, depending on my mood. I would almost certainly join them if my playing through would not appreciably improve my situation or theirs. Specifically, if there is another foursome in front of them, then my jumping between the two groups is just going to clog things up. Better to consolidate and make a larger playing group, strangers though they may be.
In short, playing alone can be very tranquil, good for your game and a great deal of fun. But going out alone doesn't mean you should expect to finish that way. I believe you should always be prepared to pair up if circumstances dictate.