The concept of "knowing your why" has become very popular as of late. There are tons of resources, videos, and step-by-step guides designed to help you identify your WHY. I will say that the overwhelming majority of coaches I know have great intentions and motivations behind why they coach. If you haven't processed through why you are coaching yet, I strongly encourage you to read and work through Joe Ehrmann's book InsideOut Coaching.
However, what I find missing from most coaches' knowledge base is why their athletes are there. It is easy, based on the level and age group you are coaching, to make assumptions and stereotype all your athletes with the same reasons and motivations for competing. Just like we, as coaches, have our own unique WHY or motives for getting into coaching, so do our athletes...and I can tell you from experience they range ALL over the map.
One of the finest skills of a coach is the ability to adapt and customize to their individual athletes. Coaching a kid who is there just to be around their friends the same as you coach the kid who wants to be a standout and make it to the next level will frustrate you and the players all season long!
Starting each season with clear expectations will not only save you headaches down the stretch, but it will lead to a more unified and higher functioning team. The link below is a simple document I have used for years with all ages, talents, and sports (some modifying required). Just having these simple answers gives me a huge advantage on how to implement my WHY as a coach in a way that will have the greatest level of impact on our team and each individual athlete. Please, take, modify, and use this tool (or create your own).