A lot of D&D parties will start out with the characters knowing each other. Or maybe just a couple of them do. But what I like is when my character strikes up a friendship with an NPC. Especially if it's a throw-away NPC that the DM created to fill a small role like giving information.
Admittedly, it doesn't seem to happen too often. DM's have enough to worry about without having to detail Sam the Street Urchin who has taken a shine to Brokass the Half-Orc Barbarian, and wants to follow in his footsteps. But, when they have the time, and it works out, it can be pretty cool.
Personally, I recommend all DM's and GM's consider having an NPC (or five) follow the party. They can be a good mouthpiece for the DM when he needs to give the party a hint, or steer them towards something that they may not necessarily notice they need to go to.
These NPC's can also be a boon to players who want to flesh out their characters. What if they want to explore their character's romantic side, but the player doesn't feel comfortable role-playing that out with another player (especially if their real-life SO is at the table)? Maybe the player wants to explore their character's (invariably) tragic backstory by taking a troubled NPC under their wing, and guiding them around the pitfalls that they couldn't avoid themselves.
Really, the possibilities are pretty endless. Characters need friends outside their party. Just like we do.