Why I can't think in anything while I'm meditating

Maybe I'm wrong, but I believe that when I'm sitting to improve myself, I have to watch my mind to see what is passing through it, and to figure out the reason for that thought and then let it go sometime. But when I'm sitting, my mind doesn't seem to work the same way as when I am not sitting. I'm just there, sitting and watching my respiration without thoughts. And when I get up and go do other things, all of the thoughts and feelings come back. What am I doing wrong?