I've been single for years, and I've been in a relationship for years too, and I know how "unhappy"
you can feel regardless of your relationship status.

When you're in a relationship you think you're better off Alone, and when alone you're lonely, sad and in need for someone to hold.
After a while I realised there are too many good things about being single that no man -or woman if you're a guy- can compare to; Starting with:

Freedom
Freedom to go out and do whatever you want to do, freedom to flirt and not feel bad, freedom to meet whoever without the ghost lurking from behind observing everything and everyone, free to not answer your phone or put up with needless conversations at 3 in the morning, free to wear and do whatever and not forget the utter freedom of being/discovering yourself.

Advancing in Career
It can be a promotion or a raise, or even a change of career, but once you get rid off the dead weight you'll have so much time to waste, and it's best wasted at work or whatever you do for a living.

Better Friends
I don't know if it's just me, but as soon as I was off on my own. My relationship with my friends completely got better, met new ones, reconnected with old ones, and somehow I believe if I was in a relationship I won't have the time to "have friendships", Everything is more fun with good friends!

Some serious Living
do you know what that means?
I didn't but now I do, it's simply enjoying life, going out, doing crazy stuff, laughing, staying out late, the only thing you check in with is your Facebook and Foursquare... Drama Free Life aka LIVING!

Self Growth
I mentioned it before; being single gives a whole lot of time to do so much, and by doing so much you will uncover so much about yourself and grow like never before.
You will have plenty of time to change, update and reflect on your dreams, goals and wishes.
Allowing you to make mistakes and learn without anyone's judgement, learning how to love yourself since no one can love you like you do and lastly seeing your true reflection and actually appreciating it.

You'll Look BETTER than ever.
I don't what's the science being this, but it is true!
When I look at old pictures I'm shocked of how much I let myself go!
I looked okay! but I also looked like I was done with it.

Because your Happy!
when being single is no longer a worry but a choice, this is when you are happy no matter what happens.
If you found someone new, GREAT!
If you remained single, GREAT!
And this is a great state to be in since you no longer have the need for someone to come and make you feel complete... there's nothing missing here anyway.