What's wrong with me?

So, I was just going through my high school's network on Facebook (don't worry, not a facebook question), and I noticed a girl on there that I remembered on middle school. I hardly knew this girl at all, I might have had one cl with her MAYBE. So, I view he profile and it says In a Relationship with Jak something...and then, for no reason at all, I got really, REALLY sad. It's weird...although this girl was cute, I had no intention of flirting with her, and I don't even know who Jak is. So, why did I get so sad? I've been single for a while, and when I get rejected, I can feel it pretty deep, but this was different. This wasn't my ususal "life's so hard, I can't do this anymore" sadness, this was more of a solemn sadness, the kind where you just don't say anything. What's going on?