Why is it

Everybody has experienced those nights where you run into someone you didn't expect to see and end up having a really good night with them. You might meet someone completely new and end up clicking with them, bump into an old friend that you've drifted apart from or just get to know somebody properly that's always been on the periphery but that you've never really been friends with. You end up dancing all night with them, or sitting down somewhere and talking about absolutely everything, really bonding with them (and not just because you've had a drink, just because you genuinely do like them) and vowing to meet up again for another amazing night, and then never do. You never even text each other. You might not even stop and chat when you see them on the street!
Why is this?
Everybody knows the kind of situation I'm talking about because it's so common that it happens to us all, so why do we constantly moan about our boring social lives or get annoyed when our friends are all busy or whatever, when we probably know about 50 people that would make wonderful friends but we just don't bother getting to know them?
Fair enough in the sixties when people didn't have mobile phones and social networking sites, but in times like this, when everybody you've ever met is just a click away, we have no excuse to be so lazy.
I'm saying this because over the past year I've probably met so many people who I've really enjoyed meeting, added them on Facebook and then we've never spoken again, and it pisses me off.
I know it's a bit late for New Year's Resolutions but a new one of mine is to make the effort with everybody, not just my best mates but everybody that I enjoy spending time with or want to get to know better, or miss being close with or whatever.
I think if everybody did this then the world would be a much better place.