What makes it easier to say hi to someone nearby?

We're launching CAT - Come And Talk tomorrow, and while building it we kept coming back to one question:

Why is it so hard to start a conversation with someone sitting right next to you — at a café, the gym, or a conference?

For us the answer was: knowing the other person is open to it, and having a shared interest to break the ice. That's why CAT shows people nearby by interests and lets them accept a message request before any chat opens.

Curious what you think — what would make you comfortable saying hi to a stranger nearby?