Before our Lord's second return, there will be plenty of signs to show that the time is drawing near, but few will heed them. Preoccupation with the affairs of this life can dull us to the spiritual realm.
The Lord kindly but seriously, points out the urgent need to be ready for His return. In the same way that a thief comes when people are the least prepared, so our Lord will return in a time when people are not looking for Him.
This is not because our Lord desires to surprise everyone and see how many He can catch unprepared. He is urging us to be watchful so that we will be prepared. Jesus gave us the signs of the end times to help us be prepared.
He is simply prophesied that there will be a condition of apathy in the latter days that will tend to lull even the faithful to sleep if they don't take heed to His words.
Believers will not be assembled on a mountaintop somewhere waiting for the Lord's return. They will still be going about their daily routines.
The Lord told us to "occupy till I come" (Luke 19:13). We should be ready for the Lord to come back at any moment, yet work as if His return was still far off.