Sincerity is not enough in this life. Some people are sincerely mistaken. In other words, the human intellect errs, at least occasionally. Honesty and transparency are not always enough to correct our mistaken understanding.
Likewise, it is not enough, in this life, to pursue our own interests and desires. Sometimes we desire something evil, sometimes we desire something good. Sometimes we desire something appropriate, sometimes what we desire is inappropriate. Desires have to be rightly ordered. If they are not rightly ordered, then listen to the epistle of James: “Then desire conceives and brings forth sin, and when sin reaches maturity it gives birth to death” (James 1:15).
It is helpful to reflect on our own desires so as to be in touch with them, first of all. What is it that you want? What is it that you really, really want, in the depth of your soul? When you come in touch with that desire, you’re not done! You still have to consider whether it is for good or for evil, for something appropriate or inappropriate, and, ultimately, whether it is according to God’s will or not. This process is why the Spiritual Exercises of St. Ignatius Loyola are so helpful. The exercises are a tool that used by a spiritual director in order to help someone who is on retreat discern God’s will in their individual lives. They are an answer to the injunction of the epistle of James: “Do not be deceived, my beloved brothers and sisters” (James 1:16).