Why did I get so lonely?

I just got back from dinner with my friends.
We had great food and beautiful bottles of red wine, and everything was so fabulous!

But before going to the restaurant, I had 30 minutes to kill.
I didn't feel like having coffee to keep me awake; I didn't need to eat or drink before upcoming dinner with my wonderful friends.
So I decided to walk.

And when I was walking down Omotesando all by myself,
going back and forth between Omotesando crossing and the restaurant,
(a lot more than twice!), I saw so many happy couples - young and old - hand in hand,
walking down the street, and smiling at each other.
Obviously love was in the air between and around and all over them.
And when I saw them, I felt so lonely, feeling like I had no one to love and be loved by.

You know, I have all that I think makes me happy!
- I am healthy mentally/physically/spiritually, and
I have a good job to keep myself healthy and happy.
And I do have great friends and wonderful family!
But why do I get lonely sometimes?
Isn't that enough?
Does this mean that I am too greedy?
Am I asking too much?

Or maybe people, whether they're single or taken,
sometimes get lonely?