When I was single I hated it so much that I was very motivated to not be single any more. That motivation was a force that has no equal in my life. It would make me do things that are almost incomprehinsible to me now, like approach strangers on the street, or go to social events where I knew noone and actually mingle and meet people. I hate that stuff so very much and it takes a huge force of will to put me into the mindset where I can do it, that only the sheer desperation of needing to not be single any more could make me do it.
I also realized at some point in my life that the best way to make yourself attractive as a single man is not to do the retarded things that most people do (work out, buy new clothes, buy cars, lol) - it's to have a good life. If you meet a girl and she comes into your life and you're doing exciting things and hanging with friends and you know the secret spots and are a regular at the cool places, that's a life she wants to be a part of. If you meet a girl and you don't know anyone fun and you stay home all the time and are generally a loser, of course she doesn't want to be in that life.
So that would make me try to have the life that I thought I should when I was single. I would try to hang out with friends, go out more, be involved in the community and what the hip kids are doing and talking about and all that.
The weird thing is I hate all that stuff and think it's so vapid and pointless, and as soon as I don't have to do it any more I want to stop doing it, but I think I am more happy when I'm doing them.
(maybe that happiness was an illusion though; it could just be because it's unusual for me, if I actually lived like that long term the happiness would go away; it could also just be the shallow happiness of social approval, feeling like you're doing what you're supposed to be doing in society; "oh yes I am being so successful at living, look at me, what a good job I'm doing"; I suppose most happiness comes from feeling that you are doing something that leads you towards a better life, it's not actually that you like those activities better)
To some extent it's that human happiness is based on a differential. So if you are basically content, then you will be unhappy because things aren't getting better. But if you are deeply miserable (eg. single) then there will be lots of moments where you think things are getting better ("I met someone great! maybe it will work out with her!" or just "I have cool friends and hobbies! I'm so desirable now, great life will be mine!") that give you hope and that makes you happy.
There are some other odd things I miss. Like being single gives you motivation to work out, shower, brush your teeth, dress well. When you're not single all those things just feel like chores, there's no reward for them, there's no positive feedback. You have to just do them because they're obligatory, and that's just wearying.
(I don't understand people who are married and still trying so hard to impress society; your self-esteem must be just awful. You see these couples that are dressing all hip and following all the trends, riding their kids around on their bike and making their sustainable home with reclaimed furniture; oh la di da we're all so impressed at how trendy you are; no we're not, your need to fit in and get approval is pathetic. That's what you do when you're in your twenties and trying to get laid, not when you're married and old. You wear sweat pants now.)
I miss the added value. There are so many things in life that are just not that great, and really not worth doing without the possibility of meeting someone. Certainly things like going out to bars and clubs, but even just walking around the city, going out to restaurants, going to coffee shops. If you're just doing it for what they sell, WTF are you thinking, it's awful, what a waste of time and money. It's only if you can flirt a bit, possibly meet someone, that makes it worth going out to public places. Even if it's only one in a hundred times, that little bit of added value pushes it over the edge from "not worth doing" to "hell I guess I have to get out of the house and meet people somehow".
I miss the way that when I was single I would try to make myself a better person, so that if I did meet someone special I would be good enough for them. I would try to be nicer and more open and more friendly and all those things that are hard for me. It was rewarding, and I liked who I was, but it's also just so exhausting that I can't keep it up without that motivation.
A very typical case is music. I used to spend lots of time following new music, local bands, checking out new things, going to shows. Part of it was because I liked music, but a lot of it when you're young is because music is a kind of identity, it's a lot like clothing, it's a way of showing your personality to the world, it's a way of defining your social group. Knowing the right bands gives you "cool cred". (I get the impression this is not quite so much the case with today's youth as it was in my generation, where grunge vs slowcore vs punk vs new wave was the most important aspect of defining the social cliques). Once you get older you realize that's all so incredibly retarded and shallow, so you stop making all that effort to impress other people. But I do miss being up on new music and listening to new things and so on.
I guess in some ways it's similar to the times in my life that I've decided I need to cut out all the simple pleasures that I believe are quite harmful to me (computers, TV, alcohol). If you forbid yourself from those things, then when you get to the end of the work day you're faced with this interminable evening and the quandary of WTF DO I DO !? You become so miserable that you get crazed and motivated to really do something new with your life, get out of the house, try something fun. It's so awful, but probably actually better for me.