I am not sure I'm focused enough to have just "one thing," but the meaning behind it applies. People need to be passionate in their lives. Something to make you want to keep going. Without passion, we are just robots doing what is needed. Without passion, a person is devoid of that spark of life, that soul, that is the very mark of life.
I haven't been alive for a long time. Duty to everyone but myself robbed me of that passion. I have been taking my life back, and in the process have been haunted by the ghosts of passions past. Some I would like to be future ones too, but that I will leave up to the future. As humans we tend to let our minds wander either into past memories or project ourselves into future events that may or may not happen. Now is all we have.
So, for now, I am demanding more time to work on my passions and friends: I joined a local group dedicated to a private interest of mine just recently; I plan on being more active in the swing dance group I'm already part of; I mentioned that poinsettia I'm looking into turning into a bonsai (if the local bonsai club's meetings didn't conflict with my schedule I'd rejoin that group); and I have year old plans for my business that got put on a shelf which I am dusting off and going to implement.
In life people get bogged down by obligations, and personal passions go by the wayside it seems. We need to constantly remind ourselves to rededicate ourselves to our passions lest we lose them to "the slings and arrows of outrageous fortune" to use a phase borrowed from another William.