Tomorrow I will start my diet
Tomorrow I will start running again
Tomorrow I will start that book
Tomorrow I will catch up with friends
The issue with tomorrow is that it ignores today. Today is the most significant portion of your life, because without today there is no tomorrow. Striving for new goals, meeting up with old friends, savings money, etc are all things that can be activated the same day as they are thought about. For example if you have not spoke to someone on facebook you have been wanting to talk to, shoot them a message, it takes 30 seconds to type a message and hit send.
Do today what could be done tomorrow because then you do not have fight regret. You have eliminated that feeling by stepping ahead of the game. There is a guarantee that you are going to feel more positive about whatever that circumstance was.
The premise being taking on challenges in the present moment Is to eliminate one more thing from being a stressor. Stressors range from all kinds of things, but whatever there size may be they add up, eliminating one at a time and ones that can eliminated immediately will potentially make life a little lighter.