Starting with "Why" is a great idea.
If you're not familiar with the concept, it was coined and made popular by Simon Sinek and speaks to the idea of finding a deeper purpose in what you intend to do.
I would think it is most popular in a business / start-up context, but could be applied to most anything we do. By starting with "why", we are expressing our deepest purpose and reason for acting.
A good Exercise Professional will take measures to help their clients find out why they truly want to exercise. Doing so has the compounded mutual benefit. It helps to build long-term relationshipss and increases the probability that clients will see positive, life-long results from their exercise efforts.
But, getting too focused on your "why" for exercise might have a negative effect. It can be difficult to find and it might cause you to have to face some real demons.
Depending on your tolerance for facing such things, your "why" might elude you. In turn, if you or someone infiuencing you has somehow made its discovery a precursor for starting your exercise, it might be an all too convenient excuse for pushing your exercise (ie. your increased health, better sense of self and overall improvement in well-being) to the sidelines.
The best thing about the "why", in my experience it two-fold. First, it is more a process of discovery then a singularly defined thing. Second, it can evolve through that process as the discovery.
If you are having trouble finding your "why" for exercise, here is on to try on, even if you don't believe it (yet): I exercise because I am worth the effort. It's a start point. Work from there.