If you have ever read a contemporary self improvement book, the place they usually start is the identification of your "why?" What's the motivating factor in your decision? Most of these books mention family, friends, financial security, health, etc. Not that there's anything wrong with any of those reasons "why," but in my mind they fall short. One only has to look at the disappointing long term results the majority of people achieve, to figure out something's missing. Could it be God? The long and short of it is "yes!"
When Ignatius of Loyola was penning his Spiritual Exercises, he noted what he thought our "why" should be right at the beginning, in a section he called the First Principle and Foundation. Fr. David Fleming SJ translated this last line using contemporary language:
Our only desire and our one choice should be this: I want and I choose what better leads to God's deepening life in me. 
Pretty straightforward, but also hard to do today. For me, I take this line to mean that my why should speak to my singular desire to choose that which deepens my relationship with God. That's it. If it/he/she strengthens my relationship with God, then I'll choose to have it/him/her be part of my why. If something or someone weakens my relationship with God, then I reject it and it/he/she has no part in my why.
I hope you can see the importance of choosing our why through the lens of faith. I think choosing this way reminds us that our choices can have eternal consequences. Discerning our why(s) is an important exercise and deserves our time and effort.