There comes several times in a person’s life when you need a shift in your focus. Normally these shifts are influenced by the need to do something like School, Keep your parents sane, Get a car, Start a business, ect…
When your are more established you might already have everything you need, so justifying a major focus change might seem unworthy.
Kind of like how moving your furniture around can give you a fresh new feeling in your house, shifting your focus can give you a new positive feeling about yourself. If you compile that with the new focus being fitness and being more happy then you have a strong formula for awesomeness.