"There are always plenty of rivals to our work. We are always falling in love or quarreling, looking for jobs or fearing to lose them, getting ill and recovering, following public affairs. If we let ourselves, we shall always be waiting for some distraction or other to end before we can really get down to our work. The only people who achieve much are those who want knowledge so badly that they seek it while the conditions are still unfavorable. Favorable conditions never come."
I found the italicized portion of this quote in my usual source, The Quotable Lewis, but I searched and found the original speech online here: Learning in War Time. I highly recommend you read the speech in its entirety.