I'm married and don't miss being single at all! I can relate to the flirting as one of the best things about being single though. For me, it has nothing to do with wanting to be with someone else or any dissatisfaction with my spouse. It's mainly about how exciting it was to think that your Prince Charming was just around the corner. When I was single, it felt like a roller coaster ride--one day, you're at the top of hill and everything's a rush. Also, it's definitely an ego boost when you flirt even if the dude is a toad you wouldn't touch with a ten foot pole.
Now that I'm married, we still flirt with each other and it's great...but you know how the story is going to end so there is not the thrill of suspense there like there is when you're single.