Why did the early Christians think Jesus would return soon?

In multiple New Testament passages the Disciples and even Jesus appear to think He would return within one generation or so: 1. Jesus says, 'What I say to you, I say to all: Watch, for you know not the hour ...' or 'Some here will not taste death before they see the Son of Man coming in his kingdom.' (Mt 16, Lk 9) 2. Paul encourages people to remain in their current states of life (e.g. celibate) in anticipation of Jesus' return. 3. The Christians waited to produce a written record of Jesus' teachings when need for it became apparent. (Such need was not immediately apparent because they thought Jesus would return soon.) I can only guess that Jesus' return and the hour of our death are equivalent, hence Jesus tells everyone to keep watch, but I don't see that this interpretation is supported by the text. Rather, the text literally suggests that the end of the world would come at any time, but then Jesus has delayed it by 2,000 years, which seems unjust to all those generations, and to us as well, leaving us in a kind of painful suspense. So what's the deal? Why did Jesus cause and allow the early Christians to be mistaken about the timeframe of His return? I did read a book arguing that Jesus was actually using symbolic language to refer to the Roman destruction of Jerusalem, but this theory merely replaces those questions with why Jesus would make His written testimony unreasonably difficult to understand.