We were created as rational people. Have centuries of believers believed Jesus was gong to return during their lifetime? Of course. I once heard Billy Graham say he truly believes Jesus will come back before he dies. I don't want to say he said God told him that, because I can't recall that detail, but he was very confident. That's a tough one for me because when it comes to Jesus' coming, I've never thought "it'll be in my lifetime." I don't know why...I want Him to return, but I know my believing Him doesn't hinge on His return while I'm living. I am OK dying before He comes, because that is the way it has been for over 2,000 years.
Jesus preached that the Kingdom of Heaven is at hand and that is obviously the most important truth. I feel that when people say 'Jesus is coming soon', they are really referencing what Christ said of His Kingdom. And...soon is obviously quite relative in God's sight.