One can be the start of something, a habit: one word written, one contact with another person, one exercise, one step, one jump.
One is far greater than none. One is achievable and can lead to bigger things, gathering momentum with success.
Today is often a day of resolutions. They can be grand, humble, practical or aspirational.
One is a stepping stone. From one you can get to anywhere and what’s easier to commit to than one?