Charles Spurgeon Commentary Luke 12:37-39

"Blessed are those servants, whom the lord when he cometh shall find watching: verily I say unto you, that he shall gird himself, and make them sit down to meat, and shall come and serve them. And if he shall come in the second watch, and if in the third, and find [them] so blessed are those [servants]. But know this, that if the master of the house had known in what hour the thief was coming, he would have watched, and not have left his house to be broken through." — Luke 12:37-39 (ASV)

Blessed are those Servants, whom the lord when he cometh, shall find watching: verily I say unto you, that he shall gird himself, and make them to sit down to meat, and will come forth and serve them. And if he shall come in the second watch, or come in the third watch, and find them so, blessed are those servants. And this know, that if the good man of the house had known what hour the thief would come, he would have watched, and not have suffered his house to be broken through.

As he does not know when the thief will come, he is always watching.