Mostly lightheared. 
But boyfriend and I have been together a year and 4 months. We are both 25.
He dated a lot before we met and before me his longest relationship was with a girl from 17-20ish.
He told me before he met me he was very close to getting with a girl who he liked on and off for a couple of years.
This girl just so happened to be with someone who was an old work colleague. I know this because i'm friends with him in facebook.
She chose my old work colleague over my boyfriend (I am very thankful for her for this because this was literally 2 months before I met my boyfriend). She became pregnant shortly into the relationship.
But old work colleague has just posted about being a single dad. I'm having a boring night and was being nosey and looked on her profile and posted about being single and someone commented about how she's better off without lying, cheating partners.
But I got a pang of "Wow, Boyfriend once got close to this girl and what would have happened if they ended up together."
It was a weird feeling.
I feel no jealousy or insecurity, it just hit me that yes boyfriebd and a life before me.
So did I though - I had just come out of a 4 year relationship before I met my boyfriend
Random thought but just a bit of lightheartedness