The reason for Jesus saying this could be that once a person is dead then there is no reason to consider them.

However the advice not to tell the living  what a new disciple was going to follow Jesus, would be to prevent the natural way a family would be to prevent the new disciple leaving home.

Jesus was convinced that the work that He required, was worth the loss of all current worldly contacts.

Where can we find this zeal today apart from the extreme moslem suicide bombers and extreme Christain monks?