Musar zu Bereschit 15:1

The tenth quality: The love of the pious man for God, blessed be He, is not dependent upon anything or given in order to receive a reward. And because Abraham sought no reward for his service to God, therefore God, blessed be He, said to him (Genesis 15:1), “Thy reward shall be exceedingly great.” For such is the way of God, blessed be He, with His pious ones. When they serve Him, not for the sake of receiving a reward, He doubles their reward and gives them a double portion of goodness. As He said to King Solomon, peace be upon him, (1 Kings 3:11), “Because thou hast asked this thing, and hast not asked for thyself long life, neither has asked riches for thyself.” And it is written after this (ibid., 3:13), “And I have also given thee that which thou has not asked, both riches and honor.”

Avraham: after he went out to assist his nephew Lot who had been taken captive, was afraid and told himself perhaps his deeds were not completely pure, as our sages explained on the verse: "do not fear Avraham" (Gen.15:1) - "Rabbi Levi said: 'because Avraham was afraid and said to himself, among all those soldiers I killed in battle, perhaps there was among them a righteous or G-d fearing person?'. Therefore, he was told (Gen.15:1): 'do not fear Avraham'" (Genesis Raba 44:4). And our sages said in Tana D'Bei Eliyahu (Rabba 25): "a person is not told 'do not fear' unless he is G-d fearing in truth".

Thus, we see that just like much contemplation is the way to acquire constant fear, so too interruption of thought and idleness of study is its greatest detriment, whether this is due to preoccupations or will - every interruption of thought is a nullification of the constant fear.