When I was single most recently, just coming out of a shaky relationship with an ugly conclusion, I felt completely drained. I felt like I never wanted to be in that kind of position of vulnerability again. Over time, by focusing on other things (work and online gaming

) I was able to restore my energy and feel better about myself.
Of course, then I met the girl who became my current girlfriend.
But my current relationship is absolutely exhausting. We have to put a lot of effort into it to make it work. We talk on the phone a lot, we spend a lot of time together, and so on. Since I met her I only get like 5-6 hours of sleep a day, and I'm the kind of person that needs 8-10.
I do agree that being single can make you feel tired (or something like a "what am I doing it all for" kind of feeling), but difficult or complicated relationships are just plain hard work. Just don't force yourself cause you think it'll solve all your problems is all I'm saying.
By the way, if girls say they "can't believe" you don't have a girlfriend, it's usually not a good sign as such. If they really thought you were boyfriend material, they'd probably be your girlfriend. Unless they were already taken, but even then. You're probably giving off signals that say you're "domesticated" (for lack of a better word), which is a trait most single girls don't like in a guy. It makes you a good friend, but not a good lover.
EDIT: Looks like Ayos already said what I was gonna say in better words. That'll teach me to read posts, even if they're long... XD
Jam it back in, in the dark.