Hanging out with happy, optimistic, and ambitious people has a positive influence on one's own disposition.

What this subject means

optimism Expecting things to go well: whether it is what gets things built, or a bias that forecasts keep paying for.

What David Heinemeier Hansson actually said

Do you hold this korrent?Do you also believe this?

Shared subject first, then similar wording. They may agree or disagree.