I see what Mr. Mill is saying in that quote, but I don't necessarily agree with the statement.  I think that he is saying that when you stop being happy to wonder if you are....then you are putting a pause on your happiness.   Everyone now and again needs to take a look at themselves and evaluate....If no one ever evaluated themselves, we would always be "leaping before we jump."  Another way I could interpret this quote.....maybe he is saying that if you get caught up in evaluating yourself, then you can never move forward.   I sometimes wonder if I have done this....probably so.  If you are always trying to fix and concentrate on your problems, it probably is harder to focus on the happiness in your life.  I think that when negative thoughts cloud your mind, it is like trying to run on fly paper.  Your stuck on the negative...and like a fly if you wiggle around too much on the fly paper (negativity) then it will lead to your demise.   These are my morning thoughts for the day...Thanks random quote!