I think we need a separate thread for people who are single and alone and another for single mothers. They're quite different things.
Bad things about being single
yes to the loneliness, what my friend calls 'background company' i.e. you can be in the same room, but don't have to talk. I told her she was welcome to come to mine on a Sunday morning to read the papers in each other's presence, but not talk, but she said it's not the same. I agree really.
Also having to face all of life's problems on your own. When there's two of you, there's someone to bounce problems off. When I'm alone I just ruminate until the problems become huge and insurmountable. Often just quite simple things like 'how can I buy a new piece of furniture if I don't have space' and I need a friend to say I can get rid of something else. I suppose I need input from someone from a different point of view.
So for both those things, meeting up with friends helps, but with some people it's hard to 'get an appointment'.
Luckily I have a lot of single friends so I don't get the problem of being the only single person around very often.
Expense - living alone, going on holiday alone is obviously more expensive than pooling two incomes and sharing things.