People who have deep meaningful conversations are happier.

Assuming this is true...

“We found this so interesting, because it could have gone the other way — it could have been, ‘Don’t worry, be happy’ — as long as you surf on the shallow level of life you’re happy, and if you go into the existential depths you’ll be unhappy,” [said Matthias Mehl, a psychologist at the University of Arizona].

... is it because deep meaningful conversations cause happiness or happiness causes deep meaning conversations?

[Mehl said that] substantive conversation seemed to hold the key to happiness for two main reasons: both because human beings are driven to find and create meaning in their lives, and because we are social animals who want and need to connect with other people.

“By engaging in meaningful conversations, we manage to impose meaning on an otherwise pretty chaotic world,” Dr. Mehl said. “And interpersonally, as you find this meaning, you bond with your interactive partner, and we know that interpersonal connection and integration is a core fundamental foundation of happiness.”...

“...Can we make people happier by asking them, for the next five days, to have one extra substantive conversation every day?”

I don't know if conversations in blog comments sections count, but go deep and report back in 5 days.