When not in a relationship, they're quite social and available. They're easy to talk to because they actually listen and care. Their minds aren't completely occupied by them and their partner's relationship.
When in a relationship, they're never available; and when they are available, they're not really 'there'. I don't know what's going on in their head, but I gather they're daydreaming about their partner.
I've never been in love, but when/if I ever am I hope that my world doesn't shrink. I still want to be friends with, well, my friends; I still want to be able to function in society. While I will love and be devoted to my partner, I will not ditch the rest of humanity, my world won't shrink to just me and him. Sure, I'd love to experience the world with him, but too often I see couples shut the world out, not enjoy it together. I know it's possible, I've seen it done.