In the professional world, “proximity” is hard to maintain, as work colleagues are reassigned or move on to new jobs. Last year, Erica Rivinoja, a writer on the NBC series “Up All Night,” became close with a woman, Jen, when they worked together on a pilot. Almost instantly, they knew each other’s exercise schedules and food preferences. Jen could sense when Ms. Rivinoja needed a jolt of caffeine, and without asking would be there with an iced tea.

“But as soon as the pilot was over, it was hard to be as close without that constant day-to-day interaction,” said Ms. Rivinoja, 35. They can occasionally carve out time for a quick gin and tonic, she said, but “there aren’t those long afternoons which bleed into evenings hanging out at the beach and then heading to a bar.”

The workplace can crackle with competition, so people learn to hide vulnerabilities and quirks from colleagues, Dr. Adams said. Work friendships often take on a transactional feel; it is difficult to say where networking ends and real friendship begins.

Differences in professional status and income also complicate matters. “It really does get weird when your friends are making tons more than you, or tons less,” said Adriane Duckworth, a former marketing executive now working as an artist in Hamilton, Ontario. She recently welcomed a promising new couple into her circle of friends, but they quickly turned people off with their obsession with money.

“At our wedding, other friends of ours who were seated with them actually complained to us afterward about the couple who was asking everyone how much money they made,” said Ms. Duckworth, 32. “People who made less felt uncomfortable discussing it, and people who made the same or more just felt it was weird to talk about it so nonchalantly.”

Once people start coupling up, the challenges only increase. Making friends with other couples “is like matchmaking for two,” said Kara Baskin, a journalist who works in Boston. “Not only are you worrying about whether the other woman likes you, you’re also worrying if her husband likes you, if your husband likes her, if your husband likes him.”