When there isn't enough text in the "flaws" box.

If you're a DM and you want to have a fun RP session for a group that's been together for a while, try introducing some sort of holiday or tradition where they're all expected to get gifts for each other. (e.g. a secret santa)

Gift giving is very personal and speaks heavily to what your character thinks and knows about the other character (or if they don't know them well, it encourages them to ask questions about their preferences and backstory) - so it's a great vehicle for getting the characters to learn about each other and bond.