Just that really. In the last couple of years have had the experience where I've been getting to know someone, got on really well with them etc, etc but then something has happened/I've found out we have really different values or something like that that makes me realise that while we will remain friendly we won't make that leap to real/true old friend.
I've got about 4 friends that I'd put in that category - 3 I've had since school and the other since we were first time mums.
(just realised that it could sound like I'm being horribly judgy and dismissing potential friends if they are not up to scratch!!
I don't mean that at all just that sometimes you realise you haven't got as much in common as you perhaps think at first. 