When creating a party for a one-off, I always put some extra effort into making sure they have reasons for being together and for trusting each other... to a certain extent.
When my players create characters for campaign play, however, they seem to approach it as a matter of filling the necessary party roles, but then each player creates their own character to fit. To a certain extent, I think this is inevitable, since some of my players put a lot more effort into backgrounds and such than the others, but I do sometimes wish the group as a whole would create their characters to mesh a little better.
I do inform my players of a small number of requirements for character backgrounds, though (such as "your character must have some reason for being in Sasserine", and "your character must have some reason to actually be an adventurer"), and amongst these is the requirement that the group must have come together before the campaign proper starts... I ran out of good ideas for "group origin" stories a long time ago.