Sometimes, you just know. But you can't explain why. Let me offer an example.
I recently completed an improv class. I had my doubts going into the class about the experience given my participation in past improv classes, not to mention where I felt I wanted to invest my energy. Nonetheless, I had a great time and met some wonderful people in the process.
A "drop in" improv group offered me and others in my class the opportunity to further develop our skills. I attended. Once. I had a good time. I laughed a lot. Granted, I had some awkward and uncomfortable moments but nothing that caused any lingering anxiety or frustration.
Yet as I drove away from the drop-in improv meeting space, I knew I wouldn't return. I knew my future lay in areas other than improv. I just knew. I didn't need to consider the matter any further. I didn't tell myself I needed to give improv another chance. I didn't need to weigh the pros and cons of improv. Nor did I feel compelled to ponder my "future possibilities" in improv. I just knew it wasn't for me. And I didn't need to justify this feeling, nor explain it to anyone else.
Upon coming to this realization, I felt an assuredness and lightness of spirit I've come to cherish. It's fleeting at best. It doesn't linger--it never does. So when I feel it, I pause, I notice it, and I enjoy it.